Thank you as that is what we need to know.
If the AC runs much in terms of hours with compressor on, you will need to have a generator to last more than a couple of days. At 10 hours of compressor run a day you would use about half of your capacity.
The norm in class b vans and most other RVs, but I have no knowledge about it they have done the Gretch differently, is that shore power is always the first option for AC power if it is available, with a generator second, and battery power/inverter third. High quality inverters will have a power boost type feature that will supplement the other two if needed to carry the load, which with air conditioners can happen when the compressor is starting, but not usually when running or by running a microwave at the same time.
I assume you would be setting it up to plug into the shore power inlet plug on the van? If so, you should make sure that the generator you get is neutral to ground bonded to assure safety. If you plan on wiring directly into the van, the priority I stated will not happen correctly unless you add an automatic transfer switch as you should never have the generator and shore power connected to van at the same time. Using the inlet plug for the generator makes it impossible to have both connected at the same time so takes care of that.
2000 watts will probably run the AC OK unless you are using other power at the same time, like the microwave or inductive cooktop, so larger may be needed if you want to do that. If the van has the boosting type inverter, it will cover that, though.
Any way you look at it, the charging is likely to be much slower on generator or shore power to recharge a 17KW bank when compared to the van engine (driving) 51v alternator charging. Just compare that even if all 2000 watts from the generator went to batteries only it would 8 hours of running generator and using no other power to do it. Shore power is 3300 watts so would be faster than the generator but probably limited by the shore power. I can't say exactly how fast the van will charge them as we don't know if they limited the engine charging max output or not. If they are getting 300 amps of 51v power from the alternator that would be 15K watts which is a big load on the engine, especially if towing a trailer. 300 watts of solar is mostly for show in a 17Kwhr system, I think, as it will only give about 1.2kwhr per day in very good sun. Using the generator to keep up with power use as it happens, like with the air conditioner, is really the only practical way to do it.
I think one thing to remember is that it is very difficult to accurately answer questions when all the information is known ahead of time. In high powered systems like yours, you have plenty of power to cause all kinds of issues and those with those kinds of systems rarely ask about things like priority or running the AC unless they are using the AC a lot offgrid. In your case having shore power in the equation is not possible because you are are talking about offgrid use and that simplifies the question. If someone answers without all the information, I would take it as unreliable in most cases as it could be based on non long term AC use in their 2002 Roadtrek with one battery or such, or just a guess.
I completely agree with the not running the van to charge batteries as that, per Mercedes, is that you have to drive it for a stretch after far less idling than you will need.
Good luck with your Gretch, hopefully you can stick around or check back in to let us know how it all works out for you as more data on here is always helpful for others that might need it. I think that systems like yours will be getting more common going forward and I also think that some will be disappointed on how long even a huge bank can keep the AC running offgrid so a very good time to get this discussion.
