Bill-they can't be converted to run diesel (many reasons-injection system, compression ratio). You can buy a diesel genset-but most of the appropriate size enclosed gensets for sale are Chinese (Yanmar clones), and they're relatively heavy, noisier than the quiet Hondas and Yamahas, and the exhaust odor can be a problem at times.
I have the Kipor 3500 T diesel genset. It weighs about 300 pounds-I ramp it into the pickup. It is fine for rest stop use-it blends right in with idling trucks. The noise inside the trailer is not objectionable, and the unit has a mellow deep sound. It starts the AC with ease-a lot more authoritively than my Yamaha 3000 boost. The fuel consumption is very low. I think diesel is safer in the bed of the pickup. But I don't use it if I'm going to stay in campgrounds where noise/smell may be an issue (I use my Yamaha for that).
As for theft-an advantage of a larger heavier genset locked into a pickup bed, with a locked tailgate is at least a little harder to steal. I cover mine with an old burlap bag, and stuff a trash bag or two around it when I'm in a marginal area. Disguise helps here.