"Salem" - been there, done that.
No fun no matter what your "skill level" is, when -
The valve is way up under the RV.
(There WAS a time when the valves were very accessible *externally* on the 3" drain pipe - as compared to cable and electric valves in use now)
There is NO room to cut pipe and add the/a "rubber coupler"..:(
Made my own "spreader".
Yeah - used the vaseline "trick".
Still a *major*, MAJOR "PITA"..:(
As a "heads up" (check first!) often the cable (or the electric motor) is the culprit and the valve is OK.
For my own peace of mind I added a basic "manual pull" valve in the exterior section of the 3" drain.
I now have the option of just leaving the "PITA" valve in the *open* position.
Lubricant/s:
Other than via the pull rod on a Valterra valve, there is no way to add lubricant to the *interior* of the valve.
(Spray & pray?)..:@
Some folks add some vegetable oil to (in) the black tank (when it's empty), add water and flush.
Does it help? Nobody will ever know for sure..:h
BTW - everyone should carry a "Valterra T58 Twist-On" valve as a temporary backup.
(About $15-20 available from Amazon & other suppliers)..:C
And of course - that requires you to get the failed valve into the *open* position. But should be lots easier than a replacement.
.