I wonder what's going on that you're going through so many valves. When you said the original got damaged, what does that mean? Did the valve get damaged by road debris, or did it just wear out?
Could the piping that the valve is mounted in be putting undue pressure on the valve itself? I guess you would know this by how tight the valve is by how hard it is to remove the valve after removing the four bolts.
What chemical if any are you using in the tanks? I wonder if something is causing the seals to swell over time and cause the valve to be sticky.
Can you see the valve and handle or is it inside an enclosed underbelly? Is it possible that something is binding up the release rod and the issue is not even in the valve?
Are the tanks still securely mounted, or could they be sagging and putting pressure on the joint holding the valve?
Sorry that all I have is questions, but I figure they might help steer you to a cure.
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2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup