It is possible that you have a bad valve or that there is an internal plumbing problem. If you are not handy enough to deal with these issues, you'll have to take your coach to an RV repair shop that is willing to do a job like this and pay them to clear the clog for you. This may be something as simple as replacing a part or as complicated as dropping your tank, cleaning it out, finding the problem and repairing it. You have to be very careful about what you put in your RV toilet. For example, if you are using regular toilet paper, that could be your issue! You don't want to damage your toilet or your tank, so hiring a pro at this point is probably your best bet.