Tom (leave Catherine out of this mess): and it is a mess to change the dump
valves. If you have to go to the work of getting to the black water valve to
replace it, replace the gray water valve at the same time (or at least I have).

But there is a remote possibility that the seal dried out while it was out of use. So,
you can use cooking oil, but I prefer to use Murphy's Wood (Oil) Soap. I get it from
Family Dollar Store, their own brand, for about 2/3rds the cost of Murphy's. Dump
your holding tank.Fill it about 1/4 full after you dump the whole bottle of the soap,
and let it set as long as you can. Might try letting a little seep out one or twice.
Maybe--just maybe!-- the seal will swell and stop your problem. (Or maybe you
have some crud blocking the seal and this gets if flushed out ??) Anyhow it worked
for me once, for a llittle while! Keep it in the tank as long as you can.

Donald