OK, a much less odoriferous situation.
Can you look into the pipe with sewer hose off and check the valve operation? Or run a coat hanger up to check ?
If the valve is open, there may be a blockage above the valve, I've heard of the plastic cut outs for the pipe connector being left in the tank after the fitting was glued on. but you would think on a 13 year old RV that would have happened before now.
If it is a blockage the back flush adaptor may still work, but you may end up having to run a plumbing snake up the pipe.
Or get some Draino or Liquid Plumber into the tank if there's any room.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.