Then, if it were me, I'd be seeing too large of a red flag, and walk away from that one. If he's not willing to let you check things out, other than in a storage lot where there's no supporting infrastructure, I would suspect he's hiding something.
You are trying to get the best you can with your money.
After all, what can you really check out in a storage lot? The 12 volt system? That's not much. Yup....the lights work, and the water pump runs. (I had one run too, but it didn't pump due to freeze damage.) The fridge on propane, and the furnace. Still leaves a lot to chance.
Roger