The customer doesn't want to run a generator.
A total loss system would suit his needs as long as the loss is low.

He and a few of his friends run a campsite out in the boonies where there are no utilities, except the propane they bring with them.

He already has propane lighting and heating, but doesn't want to spend $2200 to $3500 for an 18 cubic foot frig (ammonia type, propane heated).

If I can design a system to convert existing used refrigerators to one's he can use, I would satisfy his needs and make a little money for myself.

I could sell them for $500-$1000 (US).

I already have in mind how to take care of spent propane and have a good idea on safety valves to be used.