j-d is onto something. You were not clear that you have the following condition. Make sure you use a 30A power cord all the way to the campsite power box. If it is too short, use a 30A extension cord. Do NOT convert to a 20A cord. Menards sells 25 foot long 30A extension cords cheap. Even with a 30A extension cord, it can become questionable with certain rigs.
Another culprit could be your power transfer switch, assumed you have one. It could be faulty.
One more thought. Is there any other appliance in the motor home that could be turned to the "on" position? Something like a simple hair drier plugged into a shore-power-only outlet will rob the power to the a/c unit stealing the thunder it takes to get it going.
You do have a 2017 model. I assume you are under warranty. I would call the RV manufacture Thor Industries. Hopefully they can offer you some trouble-shooting tips while you are on your trip, rather than saying to take it in at your next opportunity.
BTW: I see Thor installed a 68 pound, 16 gallon propane tank inside that diesel Transit. Unless special ordered with a diesel generator, Sprinter & Transit diesels generally come with a propane generator. Nice to see you have great reserves.
Our 40 pound, 10 gallon propane tank serves us very well, but it supports only the fridge, stove, hot water tank and furnace. We have a gasoline generator utilizing a 55 gallon E350 fuel tank.
