1) Pump out services do not flush the tank, they just suck out any liquid waste that flows out the mast.
2) If there is not enough liquid EVERY TIME; solids remain in a pile (Pyramid of doom) at the base of the soil pipe.
3) If they charge per pump out, it is human nature to try and stretch the time between pump outs. Folks do this by not filling the bowl with water when flushing a #2. This is false economy. It leads to solids caked on walls and a deep layer of packed on sludge that restricts water flow from the dump mast valve making even more solids remain behind.
4) Once the "Pyramid of Doom" gets high enough to block the entrance to the soil pipe, solids start blocking the pipe itself; catching everything that goes down that is not water. (paper, deodorant tablets, poo, etc.)
So, what to do.
You are not going to like your options:
1) STOP using the toilet
2) Add 1 QUART of RV-Digest-It or other enzyme/bacteria based digester and then fill the tank with water until you see water in the bottom of the toilet opening (no higher as you don't want your base seal to leak.
3) Wait AT LEAST 72 hours (3 full days) to allow the "beasties" to turn that sludge/paper/poo into liquid waste.
4) call for a pump out.
5) Repeat AT LEAST once more.
6) Always use a full bowl of water to flush solids and never pump out unless full.
This should do the trick.
Unique RV Digest-It Unique Distributing