Looking at the options on a new RV and the Onan QG 3600 LP is listed as standard and the QD 3200 (Diesel) is the option. I have not been able to pin down the cost, but it's got to be 3 or 4 thousand d...
Biggest plus for the diesel is amount of fuel needed to keep it running. They are very fuel efficient compared to a LP generator. With number of hours yours has run. It would not warrant switching to Diesel.