I think you said it takes 6 hours to charge the power bank. Pretend its a 2a. Would make it about 12000mah of usable power.
You cant strap a panel to the roof. Run the wires under the engine to the battery. Plug your Adruino device into the cigarette lighter?
Not sure how often the vehicle...
Have any more info on the auctions that sell AC units?
A cassette style mini split would be so nice for a trailer.
https://www.pioneerminisplit.com/collections/mini-split-ductless-systems/products/pioneer-12-000-btu-21-5-seer-8-way-compact-cassette-mini-split-air-cond-heat-pump-system-full-set