Sounds to me like you have TP and "STUFF" settled around the discharge portion of the tank and has compressed causing the clog. The cap with shutoff valve/hose connection and blasting it is a good idea but the mess you may end up with may not be what you bargained for.
So if you don't like that thought call a Portable Sani-Can Company like United Site Services and ask what they charge to come out and vac your RV tank waste tank. I used to pay $55 for the service.
Once you have gotten this resolved get rid of the cable valve and replace with the standard "T" handle valve.
