jml, is your dash air is marginal now?...I doubt seriously you will gain anything but a lot of lost time/effort.
So many of these RVs have what is known in the industry as "aftermarket", after thought", "afterbirth" AC systems. Most all have been done in the cheapest and easiest method....due to the RVs having supplemental HVAC systems.
Dash air inefficiency is a common complaint with Class A's
Before diving in head first...can you do yourself an experiment? Use an extra fan to blow AC from the dash vents to the rear when driving and see if you gain anything.
Most RV AC's are undersized to begin with and or combined with a shoddy install....tend to be lack lustre in performance.
Stick a good thermometer in a center dash vent and go for a ride...run blower speed on high....monitor the outlet temp.....if the temp is not low 40s to upper 30s..I feel your wasting your time
96 Dynasty 36' that does NOT wander, thanks to RuppParts.com and their superior "poly" suspension bushings