While the above post is fairly accurate, to say there were not any emission controls before 2007 is not true. Yes, there was a big jump in the complexity of engines made in 2007 but there was a fairly large one for 2004 engines. I made sure my 04 DP had a 03 engine and if it had a Cat engine that would have been a very good decision as the 04 and later Cat Acert engines have had lots of problems. Fortunately I have a Cummings engine and today I would be happy with a 04-06 engine.
Monaco bought up a bunch of 06 engines and I have seen them in even 09 Monaco DPs. If the big thing that looks like a muffler but it has wires going to it...it's a 07-09 engine. I agree DEF engines have proven to be a good thing with the increased mpg paying for the expense of the DEF.
Unless the OP wises up and decides to go with something longer than 36' he does not need a Diesel engine.