We had a 1999 with the same chassis and it had the same problem. The problem is Ford does not insulate the cab or fire wall and neither does the coach manufacturer.
The best solution is to remove the engine cover and also pull the seats out. Pull all of the carpet out...it is probably glued to the floor and has ZERO insulation.
Go to J.C. Whitney
and order the 3/8" thick aluminum backed fiber insulation kit with the spray adhesive and foil tape. DO NOT bother with the foil bubble wrap stuff sold by Camping World.
After the seats are out and the carpet is out, cut the insulation to fit, leaving about 3/4" around the edges to get the engine cover and door sill plates back in place. Use the spray adhesive to hold it in place and refit the carpet and seats.
For the engine cover, cut and fit the insulation on the engine side with the foil side out. Seal the edges and seams with the foil tape. Now with the remaining insulation, cut and fit where you can reach under the dash against the firewall.
It took me about 5 hours to do ours. The insulation also helps as a sound deadener. I ordered some extra insulation and lined the generator compartment and helped to deaden the sound from the generator.
Ours was so hot it would melt things left on the cab floor and my wife could not put her leg against the engine cover. After this modification, the cab was much cooler and the dash air would actually cool the front of the coach.
As for the A/C ducts blowing hot when not on heat, you have a damper door not closing properly in the dash or the heater water valve is not shutting off. The easy fix for the water valve is to buy a water ball valve, two pipe nipples and two hose clamps. Cut into the hose that feeds the heater core, install the valve and close it during summer travel and open it for winter travel.
Best of luck with the mods.