I'd not carry a gas can in the trailer, if I can help it. I'd consider a RotoPax mount and have the cans mounted on the exterior somewhere.
As for a roof vent, make sure your battery on the trailer can handle it. I'd personally go for a Fantastic Vent model that has an Ultra Breeze vent cover, so the fan can be used in the rain. I'd also consider adding a vent or two in the trailer to create an airflow for incoming air... perhaps with a filter in place so the rig doesn't get dusty.
The Fantastic Fan uses about 2 amps on high, so you will need to find a way to replenish the 48 amp-hours per day used. At minimum, if you can get a 200-250 watt solar panel and a cheapie PWM controller, that would offset the battery drain.
A definite fix, but it takes some time and plumbing know-how are HepVo waterless traps. I learned about these from a friend of mine who is a plumber. For RVs, these not just replace the P traps, but the air admittance valves. Use these for the sink and tub... no more stink from the gray tank. For the black tank, make sure the toilet's ball valve is sealed (olive oil), and that cures that.
Of course, it doesn't hurt to make sure the vents are unobstructed.
+1 on Trik-L-Start or Amp-L-Start. I'd also consider a Battery Minder because it not just keeps the battery charged without boiling them dry, but desulfates them as well.
One side note, the OP should check to see if the rig has a single state or multiple stage converter. If it is a single stage, it should be tossed and a multiple stage put in ASAP. This will go a long way in preserving battery life, since a single stage will boil out batteries over time.
One extreme that would solve the problem for good would be to replace the traps under the sink and bathtub with HepVo waterless traps. These not just replace P traps, but they also replace air admittance valves.
Those will go a long way in ensuring the sewage smells from the black and gray tanks are long gone.
I also would consider a Camco cyclone vent cap or a Ultra-Fab mini plumbing vent (which uses a small solar panel and a fan.) Either of these will actively pull gases out. The Camco has better reviews, and is better if there is -any- wind at all, but the other brand will work if the sun is out.
When I am away from my RV for more than just a daily excursion, I turn the water off at the spigot, and turn the pump off. It is simple, and if I do get a leak, damage is mitigated.
I do like the idea of a manifold, but the more connections, the more likely leaks will happen, especially with all the vibrations that RV plumbing has to deal with.
I'd upgrade to PEX though. There are many different ways to add fittings onto it. I'm partial to ProPEX, but the tool is not cheap to expand the fitting ring and the pipe. Flair-It fittings seems like a decent compromise, especially if one isn't doing professional plumbing.
What I'm curious about is if the 10 year has influenced buying decisions. I do know it has affected what I chose to get... although the used rigs I looked at when before I bought my current TT were in pretty sad shape.
I hate to be a fly in the ointment, but the biggest issue I've found with factory direct rigs is that no local place will do warranty repairs, and any types of repairs (even if out of pocket) will be put on the end of the line.
Don't forget sound insulation. As per Onan spec, you should have insulation on all sides (perhaps the door) of the compartment, but not beneath the genset (so fuel dripping on it doesn't cause problems.) You should also have a vent in the front as well.
It also can't hurt to rubber shock mount everything (exhaust pipes, fuel and electric lines), and use the Onan resonator. This will quiet things down remarkably.
A few weekends ago, I was seeing 20# propane cylinders, new, at Wally World in Elgin for $20.00... empty, but purged and ready for filling.
Here in Austin, propane refill shops are few and far between, and tend to not bother with you unless you sign a contract of 250+ gallons per year. The U-haul places have propane filling stations... but usually nobody trained in their use is on site. So, for tanks, the closest place are the RV dealers. Propane exchanges I can find 24/7, and the clerk only has to turn a key, open the cage, grab a container with a blue seal on the end, shove the one brought in the cage, and stick the lock back on.
Last year was the final year that VW has made parts for the van chassis. This makes repairs extremely difficult barring custom blueprints and a machine shop.
Want a Rialta? Buy a Winnebago Trend instead. Same Euro-style, but on a supported chassis with easy parts availability.
Texas as well. In the past five years, virtually all picnic areas, turn-offs, and rest areas have been at least had their signage removed, if not completely shuttered. They have at most a few "safety rest stops" on interstates.
This is sort of befuddling, since crime is lower than it was in the 50s and 60s where rest stops were quite commonly used. I'm guessing Wally World has replaced rest stops?
You definitely did the right thing. They have/had extremely good fuel economy (22 MPG is what I've read)... but they are very slow, and trying to accelerate on a modern freeway might be a hazard. I used to have a Toyota mini in my college days... and it was almost dangerously slow, especially getting on any freeway and properly merging. Was a quite reliable truck though... those things are still in use in a lot of countries.
I would never buy a Toyota Hilux based rig though, namely for the above reasons, as well as parts availability. They are so old, you can't even hit a junkyard to pry off a replacement part, and the 4-6 cylinder engines have long since been sold for scrap, so replacements of that and transmissions are hard to find.
If looking for a small class "C" in that price range, I'd consider looking at used Cruise America rigs. They have been used and abused... but they have a solid repair log behind them, and you know what was fixed before you get the unit.
Anything on a Transit platform, IMHO, is a good thing. I'm hoping that RV places both offer "euro" RVs like the Trend, as well as "traditional" ones like the View with a slide-out.
Since this is a cutaway, there is (as of now) no EcoBoost engine model available, but the I-5 diesel is no slouch either.
What is a good variable speed pump brand? I know ShurFlo sells a small (~1 gallon) accumulator tank, which is small enough to not take up much room... but large enough to minimize a normal pump's cycle.
I'd say the tank and/or a variable speed pump is a must have, just like having a multi-stage converter.
Oh, don't forget a pressure regulator. I'd like to find one that can be inline, as opposed to connected to a hose, just so it is one less thing a skulker can make off with.
It really depends on the model. I've seen some models with bunk beds that were easily removed without damage, and easily replaced (so the next owner would have the same functionality). Just took a screwdriver and two people to lift the bunks out. Other models, the beds, once out, are out permanently, so replacing them would require a remodel.
If length were not a factor for me, there are some 30-31 foot "C" floorplans that have two bunks by the master bedroom, and if those were removable, it would be an ideal spot for a desk/half-dinette and chair.
I have to agree about the feeling that older fridges will last longer than newer models. For example, a friend's RV has an older fridge that has a pilot light, and does not need electricity in any way. Newer fridges have to keep the control board powered, which is a fairly common item that fails.