I agree that unless you build up the ability of the transmission, then adding horsepower to the engine might be a bad idea. I have a 1997 Ford based Bounder, and it has sufficient power, gets 7 MPG, and will climb all but the steepest 6% grades at 55 MPH. 6% grades are down to about 45. Not unreasonable for this model year.
I have heard that synthetic transmission fluids will cause the transmission to transfer the power better, and less loss through the transmission. However those who are producing much more than the stock power will find transmission durability a problem.
I installed a Banks shift kit when my motorhome was about 1 year old. This system caused the motorhome to shift out of overdrive less often, and to hold drive longer when climbing in the hills. Before I had to keep the throttle very steady to keep it in overdrive when going down a 2 lane road, where there might be dips and short rise to come over, if on cruise control each short uphill section would bring it out of overdrive for a few seconds, then it would shift right back into overdrive again.
Have the stock exhaust headers been changed to something stainless steel? This can make a great improvement in performance, and also get rid of the cast iron manifolds that tend to leak, and also pop off the exhaust bolt heads due to expansion. I installed a Banks exhaust system, Thorley and others also make stainless steel headers, they should perform as well.