The issue here is heat disposal.
While replacing the fridge with a propane one may seem a good idea, finding one that had a way of dumping all that heat to the exterior of the unit would be problematic.
The cooling coils are on the back remember and the airflow needs to start below the unit and exit the top.
Might be "do-able" if the exterior fridge was not in a slideout by cutting large rectangular hole in the floor below it and building a chimney flue up to the roof; you would still need to be able to access the rear for service so a hatch inside would be needed. Additionally, you would need to be concerned with the fire hazard as a "hard light" could cause a fire you would not be able to see in time to put out.
So, no. That is why you won't see this offered.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW