I noticed a post some time ago where a guy said he drilled a 1/8” hole in the shoulder of the dump valve above the handle. He would force machine oil into this hole periodically to lubricate the valve. He then screwed a small screw into the hole afterwards. I wonder of doing this may help you get some lube in there to allow you to work the valve.
Dump valves
in General Motorhome Discussion
Posted
I have done this procedure and has worked for me