RE: generator wouldn't start
Here's what I think might be the issue. First, I would say that your coach batteries are dead, for whatever reason, perhaps some parasitic drain or they're just getting old and not holding a charge. At this point, your battery isolator would be disconnected, to prevent whatever load on your coach batteries from draining your engine battery. After starting your van engine, the voltage goes up over 10.5 volts, which is the typical cutoff voltage for the isolator. With the voltage above the cutoff voltage, the isolator couples your coach batteries and engine battery together, effectively on the same circuit, and you are then able to start your generator from the engine battery.
You should do a couple of things: Check the electrolyte levels in your coach batteries and also have them tested. I've read that some of the auto parts places will test them for free, but I don't know if that's true or, if so, which ones. Also not sure if they'll test them in place or if you have to pull them out and take them in to be tested. Perhaps someone can respond to that. Secondly, check for parasitic electric loads in your system. Remember that your CO and propane detectors will have a draw on your system which, over an extended period of time will drain your batteries.
Hope this helps.
RE: '06 Sprinter? Or '07 Sprinter?
Have to agree with the others who suggested the '06. I have an '07 ('06 chassis) RT RS Adventurous with 53K on it and it's been great. Consistently 22-23 MPG and has been as high as 26. No experience with the newer, six cylinder, engine, other than to read the mileage is down in the 17-18 range. The specs say the five cylinder is rated at 158 BHP, and we have found that to be more than adequate, even when towing a 12' tandem axle utility trailer with a pair of ATVs in it.
RE: Running your generator
For that short a load time and wattage, do you have enough juice left in the batteries to run the converter?
What good would that do? I'm sure you meant Inverter right? Converter=120V to 12V, Inverter=12V to 120V. Keep em straight man.... :)
Thank you, Mr. Engineer! I guess we all knew what he meant.