Sometimes the disc from cutting out the opening for the 3" pipe can be left in the tank and will plug the outlet. Who knows, since you've had the TT for a few years, maybe "someone" put something into the tank they shouldn't have.
After having an in-tank flusher, I have found that it works much better than a back-flush elbow, but you want to use it to clean your tank (when dumping at the CG) to get the tank clean before you head home. I only use our elbow now to watch what's coming out (and to provide an angled transition).
The solution is prevention. Add 4 or 5 gallons before you use the tank, use lots of water when flushing #2, and fill the tank up before you open the valve. Resist the temptation to skimp on water to extend the time before you need to dump the tank. If you delay cleaning it properly after each camping trip, you can make emptying and cleaning a real problem down the road. Again, you want to make sure it's thoroughly cleaned before you head home.
We have had trouble in the past with extreme difficulty in emptying our black tank. Some tanks are simply harder to dump and clean due to their dimensions and configuration. Some tanks, like ours, are shallower and long and harder to drain. After opening the valve, I run the tank flusher for 5+ minutes (while breaking camp) then fill the tank up (with foot valve) and dump. Then I look down the toilet and if the tank bottom is not spotless, I repeat the rinse, fill and dump process again. Takes me 2, or maybe 3 repetitions to get the tank spotless. The tank in our first TT was a piece of cake to dump and we never ever had an issue.
You might try getting a stiff, hard-walled garden hose and pushing it up into the tank and down into the tank to try and break up the solids and clear it. A plumbing snake might work in through the outlet.
If all else fails, you can use liquid drano. Our owner's manual actually says to use it if needed and we did use it once. It MUST be the version that does not attack metal and DO NOT ever dump it into an on-site septic disposal system or you can permanently damage it. If you use this, you'd want to tow your TT around a bit to slosh the contents around inside the tank. Your tank will end up clean as a whistle.
I am of the opinion that using the correct type of TP helps so that it breaks down when in the tank. Some RV brands of TP actually do not break down as you would think they should. Some ordinary household brands do break down, like one of the Costco brands we use. You can put a few sheets of your TP in a glass of water and see if it disintegrates into small pieces after a few hours.
Good luck and report back when you get it figured out.
ps: It helps to avoid long run-on paragraphs (like in 1st post) when posting and break your point/thoughts up into smaller paragraphs. Gets hard to wade through looong paragraphs. ;)
