air bags are the easy way to fix this have air bags on rear of my 1985 class c 26 ft, ck to see springs in safe condition ,bushings not wallowed out, bolts tight.
install air bags, these will go between the rear end and frame, with individual inflation lines this very important to balance the weight side to side, set to level when loaded and avoid the temptation to air up higher, will ride bad and handle worse if you do,made this mistake one time.
as far as the bags being aired all the time almost all big rigs run air ride and the trailers also the bags are made for this purpose so isn't a problem.
also very important don't over load your rig since it won't squat anymore the bags will handle more then the rig will safely.
mine came with them on already and aired down it rides to low,replaced the schrader valves that is the small valve in the end of the line, just like in your valve stems on your tires, aired them up stayed up for 10 months so far, best thing since sliced bread.