Rick,
My first question is "Did it flush okay when you first purchased the RV?"
If so, then I would say that you definitely have a clog and your tank needs to be flushed throughly. If you do not have one, then I recommend you purchase a flexible flushing wand such as this.
http://www.campingworld.com/browse/s...s/skunum=15716
Use it while connected to full hookups to go down into the toilet line and flush the crap out of the tank.
I also recommend that if you do not have already, is to purchase a clear adapter to connect between your sewer line and hose so that you can see what is actually coming out of you tank. Flush your tank many times by filling it almost completely full and then dumping it. Solid wastes do not flow out of the tank when it only has a small amount of water in it.
If you see a lot of toilet paper, then I would recommend that you change toilet paper. I have found Angel Soft to be the best TP since it breaks down better than any other, be it RV TP or household TP. It breaks down so completely that it is barely noticeable when the tank is dumped I have found. Yep, I watch what comes out of my tanks with the the clear adapter.
Hopefully by doing the above, your problem should be resolved. I also do not dump our Black Tank unless it is at least 1/2 full so that the solids drain out and I always flush the tank many times when it is dumped with the flushing system.
Ron