EDIT: Problem solved!
Hi, I no longer own a fiberglass camper -- had to sell it and later bough a cheap used popup -- but this forum was so helpful for me when I had that Scamp
that I thought I'd ask here, especially since the tank/valve in question looks almost exactly like the one on my old Scamp
The previous owner never used the water tank so I expected to have problems. But I put a little water in it to test and the pump worked like a charm! Ok, I said, I just need to disinfect the tank and I'm good to go. A little bleach, and I fill up the tank and let it sit. So far so good.
But when I went to drain the tank, it won't drain. Here is a pic of the valve (this is inside the camper, underneath part of the dinette, with a drain tube leading from the valve out the bottom of the camper):
I know the brute force solution would be to hand pump all the water out then replace the whole valve, but I'm wondering if there are any simpler solutions or things to try.