Not familiar with your rig....but toilet backups can happen from various reasons...most common is poor design / shape of the drain line heading to the holding tank...I have seen some twist and turn with too many joints and direction changes....any of which can get a hang up...our build up of waste...causing problems....If it worked well originally then a build up some where could be the issue..."snaking" this line may be your only option to get things moving again...If you are not familiar with this procedure...get a tech to do it...as damage can be done easily....
Remember to flush with plenty of water....Always....so the line is properly cleared after use...proper TP helps also
Wes & Denise
2015 38RL, Full Body Paint, Slide in Slide, full Laundry, RV Fridge, 5.5 Onan, Magnum Inverter Charger, Samlex Solar, Trav'ler Sat System. Heated Tanks, R.O. Filtration, Sailun Tires**2016 F-450, Reese 25K Elite