If you are going back to 2006, look at Monaco's from that time frame. The top-of-the-line Signatures, Executives and Dynasty's were beautiful and would be DPF free. The more you go up the line, the bigger the engine.
Personally, I would go with an older Monaco vs. the older American Coach if you are comparing apples to apples...... I like Cummins better than CAT. It might be that a Signature could have a Detroit Diesel installed (plus).
I'm going to say it, I would not want a Maxx-Force engine. Why would you want that when you can have a tried and true Cummins.
MM.