Prevost used to use Detroit Diesel exclusively until Volvo bought them. Of course I would vote for a Cummins engine and Fruitliner ( sorry Mike :-laf ) does make a good chassis. The smaller motorhomes in general have the rear radiator and you can't see the engine at all. The larger ones have a side rad and you can see ( and service ) the engine from the back, to a certain extent of course. With the HPCR engine a front engine unit isn't as bad as years ago, but you do lose floor space. Also check out these :
http://www.renegaderv.com/.
From an engine point of view the IS
B of course is similar to our Dodges, either a 5.9 or a 6.7. The next size up is the IS
C which is a 8.3 litre, then the IS
L which is a 8.9 ( 9 litre) then the IS
M is an 11 litre and the IS
X which is a 15 litre and can go a high as 650 HP. They don't hold up traffic

If you want one with a built in exhaust brake then you start at the ISM, but if looking at an earlier version, make sure there is enough room to get the valve cover off as when they first started adding the brake there wasn't enough room because it adds several inches to the height of the engine. You don't want to know what it costs to do a valve set on one of those.

You want one with enough power to tow a toad ( or dingy ) unless you get one with a garage but I don't think that is your style.
Onan generators ( a part of Cummins ) power most of the RV market in general. You can get an 8, 10 or even a 12 KW unit in big coaches. In a diesel unit they will most likely be diesel powered as well, but I have seen some cheaper units use propane powered generators.
If you have any questions about Onan or Cummins RV engines please feel free to contact me. A model or ESN would be helpful. While I'll be the first to admit I don't know everything I do have a little bit of experience with both of them. David