I've owned many RV's in the past 50 years. They included pickup campers, bumper pull trailers, a large (45') fifth wheel trailer, and several motor homes.
In 2005 Monaco made many changes to their Diplomat, including a fourth slide (in the 40PDQ model). That opened the interior up and resulted in spacious mobile living quarters.
Monaco's choice of a ROADMASTER chassis, (with 8 airbags and 8 shocks), gives this coach a stable, quiet, comfortable ride. I previously owned another brand (comparable cost) and it rode like a "lumber truck". The way the newer Monacos handle crosswinds is really impressive. (2005 & newer)
One shortcoming of most coaches is a lack of storage. I can honestly say that this coach has more storage that a person knows what to do with! Cabinets everywhere! Many drawers! Under coach storage that most coach owners would "die for".
I've read posts authored by RV experts, and very experienced owners. Every one that I've ever read said something to the effect of, "YOU CAN'T GO WRONG WITH A Diplomat 40PDQ." I agree wholeheartedly!
Note: There is a vast difference between various floor plans and various year models. Prior to year 2005 the ROADMASTER chassis used by Monaco, and other manufacturers, had handling problems. Those chassis models (RR4R) had only four airbags and four shocks. That was insufficient for a coach of 40'. Various remedies and aftermarket cures were attempted, to no avail. Stay clear of any brand or model riding on that chassis. If you will insist on ROADMASTER RR8R Chassis, combined with a Cummins ISL Turbo Diesel and an Allison World Transmission ... You'll be ONE HAPPY CAMPER for years to come!
One last thing: Don't get hung up on horsepower. The ISL350 hp, and the ISL400 hp Cummins Turbo Diesel are basically the same engine. The 400 is simply programmed to drink more fuel, thus the extra horsepower. The 350 gets better gas mileage (diesel mileage). Any authorized Cummins dealer or authorized service center can change the horsepower (either up or down) by reprogramming the fuel controller. A local dealer quoted $170 to reprogram mine but I've heard other quotes of up to $300. My advice? If you buy a 350, leave it alone. Drive it a while and I think you'll find that Allison six speed transmission provides all the power you want, or can use. However, if you're doing a lot of mountain driving while towing ... and you don't object to loosing a mile per gallon ... have it changed to 400 hp. If you do change to 400 and you decide that you made a mistake, no problem ... have it changed back!
Monaco got it right on this high quality coach!!!