I would either use a transfer switch to power the second panel off the power inlet, or the primary panel. Or better yet, only use one panel, and label the brakers. Switch off whatever you don't need if power is short. If you plan on running off a generator, some sort of amp meter would be helpful.
A single 4kw generator is only 33.3 amps, either plan on larger generators, or cut back on the brakers.
I read about 1/2 of your post, and got a little confused at the hook up two generators to one connection might be safe. Generators have floating neutral, so either the "Hot" circuit breaker or the other wire might be 120 volts above earth ground at any point, and running both generators into the same connector does not seem safe. It might also decompress all the smoke out of the generator or the entire trailer all at once.
Anyway there are a number of safe and sane ways to get this done, probably easier than you think. Since everything is plugged into receptacles anyway, it is fairly easy.
My first thought is to install a 50 amp power panel with either a 50 amp cord on it, or a 30 amp cord on it, then power this from either generator #2 or the main panel if it happens to be plugged into a 50 amp service.
Then the main panel will have a 50 amp cord to it, and adapters for use on a 30 amp RV service when in a campground, or on generator #1. It will also have a 30 amp circuit breaker to feed panel #2 at anytime it is desired, with consideration given to the panel #1 and #2 can not exceed the limits placed by panel #1 input power, being either the generator or the 50 amp service, or 30 amp shore power, or 15 amp shore power.
You can also use the twist lock connection found on most generators to power panel #1 or #2, and use the plug in connection to power the other panel directly from the generator.
I would suggest small electrical panels, not the 125 amp found at most home improvement stores, and great in a garage with 16 X 28" space available for it on the wall. Perhaps panel #2 can be as small as 8 circuit breakers in it, including a 30 amp main, and the other things it powers as "Non-critical".
Do you plan on using LED lights? What about a solar panel?
What will the local health department have to say about his kitchen?
Has he run a commercial kitchen before, and been inspected by the local authorities? That matters, because they normally do not allow things like a single sink to wash dishes, not even a double sink, and they will require hot water at a certain temperature, and do not allow formica countertops. Any cold food measured above 41F (that must be kept cold) will have to be disposed of if found by a health inspector.
I do not know how the local authorities feel about a ice system for keeping food cold, and stainless steel refrigerators are very expensive. Normal home refrigerator with plastic liner inside it will not pass a health inspection, and all the food inside it will be considered contaminated. This is because bacteria can live on a plastic surface, but stainless steel can be sanitized with cleaning solution to kill all bacteria, and bacteria can not be absorbed into steel.
Congratulations! You just managed to make what should be a very simple job of installing a 50A RV service and a generator transfer switch and make it extremely complicated and unsafe. I really don't understand the need for separate panels, separate generators, etc.
A 50A panel, a 50A transfer switch, and a generator large enough to handle your loads should be easy to design, install, and operate and it will meet code.
By the way, I'm an electrical contractor.
I to was wondering if your FIL had a viable business plan. I recall that there was another thread on a similar venture several years ago. Perhaps there is some good information in that thread. Search the archives.
Congratulations! You just managed to make what should be a very simple job of installing a 50A RV service and a generator transfer switch and make it extremely complicated and unsafe.
Hah! Buddy, you just described my exact reaction when he started talking about this. Lack of "a generator large enough to handle your loads" is the sole reason for the rest of the mess. Fortunately for everyone, this scheme is now dead. I visited the build site today to put eyes on the beast, talk things through, and start planning the job. It turns out that he's been shopping.
He bought two generators, alright -- a trailer-mounted diesel rated 25kW, and a frame-mounted gasser that makes 8kW. There's no kill like overkill! The small one has a 30a 120/240v outlet, which is enough to run nearly everything, but it's meant to be mounted on the tongue to power refrigerators while on the road. The big one will run everything and then some, probably at a bare idle.
I'm hugely relieved that I don't have to do anything "creative", it's going to be a vanilla 50a panel after all. We still need to group our critical loads to one leg, for those times when only 120v shore power is available and generators aren't allowed. The power inlet will be a normal 50a 120/240v, with a normal dogbone for plugging into a TT-30R. I'll make a custom cable for the 8kW generator.
To answer some other questions:
Yes, the man has experience. He ran a profitable sandwich and ice cream joint for many years, and probably would be still, if not for his ex-wife's divorce lawyer. His dream retirement is going full-time RV and camp hosting, but he can't afford it. This grub wagon is the 2nd choice, pretty much retiring to a full-time job, but with travel and RV life.
He's got a freezer and three refrigerators (commercial stainless), and a plan to keep them running 24/7. He's got four sinks planned (one handwash, three dish), and an 11-gallon RV water heater (which IME makes water nearly hot enough to initiate hydrogen fusion).
For code inspection matters: since this isn't a fixed structure, pretty much no building codes apply. The only interested party so far is the department of health. They don't really do mechanical inspections, but my FIL knows how to build his kitchen to meet health codes. But, as mentioned, insurers and fire marshals will have their wants. So for that, but mostly for our own peace of mind, we aim to do all of the electrical and gas to meet commercial code.
On the 12-volt side, we're doing LED running lights, but the coach stuff will be economy grade. Some scare lights and porch lights outside, a few fluorescents scattered inside, and a vent/light in a tiny wet bath. No solar planned, there's not much they can serve without AC power, so the batteries will always be topped up from the converter.
It'll be fun! I'm cautiously optimistic. Even if I get into the weeds and end up hiring an electrician, that too will be a useful experience.
By putting all the "critical" loads on one "leg" you may be creating an unbalanced load.
Yes, that can come back to bite you. I'm accounting for it in my panel design.
I'm putting the critical stuff all on one side of the panel, the side that gets power with a 30a120v adapter, for those times when that's the only available power. All the other loads can go on either side, and I can move them around to achieve balance. The non-critical list accounts for almost 70% of the total, so it's pretty easy.
This also means that when on 30a power, we may need to open the non-critical breakers on the hot side of the panel, or just refrain from using that equipment. Tripping the shore breaker a few times should provide the needed lessons.
I see how a transfer switch could help here, but I think we'll wait and see if it's really needed.
A proper adapter should power both legs, so I would balance the load first, then simply label the non-essential circuits to be switched off. Of course, you could put the non-essential stuff on a sub panel as well, and simply switch it off with the main sub panel breaker.