Vern,
You may want to consider going to a big Cat truck dealership and have them run a dynamometer test to measure the HP and Torque that the engine is putting out. Is the engine meeting spec and performing as expected or not. You are comparing one coach and engine to another where the weight is likely quite different. I love the power of my 2002 C12, but I am 40ft long, just two slides, and aluminum frame in this era Beaver. I weigh 36000lbs loaded for travel. The later Monaco coaches are steel framed, bigger, and heavier.
Those modifications you mention might possibly yield some improvement but many times the muffler changes just make you feel you have more power with the added exhaust noise. Seems like you need some measurements and some benchmarks to see what might be realistic and where your problem originates.
The C12 and C13 have the same 1650 ft lbs of torque rating. 505HP on the later C12 vs 525 for the C13 so not much difference between the two. This Cat link has a chart that is pretty revealing regarding weight and HP on page 6. The chart uses a 6% grade and calculates the HP required at various MPH. At 50MPH a 34000LB coach requires 340HP at that grade while a 52,000LB coach requires 500HP. Weight may be the biggest player here, basically a bigger heavier coach and the same engine output. Weight really consumes HP on grades.
http://prevostcommunity.com/PDF/Motor%20Home%20Fuel%20economy.pdf I'm not trying to imply that you do not have a real engine issue, you may, seems like that is the question that needs an answer. It is really hard for any owner to know if you are unhappy because expectations are too high or our engine output is actually problematic and too low in a case like this.
Perhaps adding an afterburner to the rear of the coach would bring a smile while doing those hills :-)
Later Ed