The problem with the Onan diesel genset is I believe it is about 2-1/2" deeper than a propane generator. It may not be impossible but it could hang down terribly low as I think the propane generator is already one of the underhanging low points. The 4x4 Sprinters are going to be 3-4" higher so maybe then diesel would make more sense. I do know of one T1N model DIY Sprinter that has a diesel generator.
I don't know if Advanced RV has ever installed a diesel generator. I don't recall them ever saying so.