First off, I REALLY wish class C manufacturers used quad cab or super cab Fords for these, but oh well. We are stuck with $400k Earthroamers or $200k Tigers. But I digress... I have only been aro...
Yes, there are normally seats in the back with seatbelts. You should do a lot of thinking before you convert to diesel. You should be interested in how many miles it takes to break even on cost unless you're just doing it to have a project.