Often it is because the tanks are not clean enough. Try rinsing the holding tank. Some RV's have a rinse fitting on the holding tank. If your's doesn't you can buy one from Valterra, called No-Fuss Flush.
Alternative, you can buy an adapter that goes on your holding tank flush port from Camco called Dual Flush Pro (make sure you get the one with the 3" valve on it as that one works the best).
Alternatively, you can use the ice trick. You leave the tank about 1/3 t0 1/2 full or so, then dump a bunch of ice in the toilet just before leaving on a trip. The ice will bang around the inside of the holding tank as you drive, hopefully dislodging anything sticking to the sides.
The only thing that works on a RV is the owner...