I thought I would post what I found, as I have not seen this same issue posted. As far as I can tell, this was built this way and never worked.
We purchased a used 2018 SOL-375 this spring and Gray valve #1 would not close.

There is a very good link (not allowed to post the link) that was key in understanding the Lasalle bristol valve:
search: Fixing Stuck 1.5" Bristol Gray Valves ( 315RLTS ) on the unnamed site.

Problem: With the Dump "T" handle pushed in all the way "Closed" the valve was still staying open over 1".
Solution:
1) Drop the underbelly panel below the tanks
2) Disconnect the cable from the valve (follow the link mentioned above)
3) trim (adjust) the dump cable jacket
4) Reinstall
The attached picture shows the original and the modification. The cable wire should be long enough to reach the line on the tape (which also aligns with the last hole in the plastic body).
Click image for larger version. 

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Hope this helps someone.