I know this is an old post, but I've been using a Valterra twist on gate valve for years because our black tank valve leaked.
I finally took down the coroplast to fix it. I found the rubber valve seal had been crushed by the sliding gate. I replaced the seals. The factory valves are Bristol valves BTW).
Pretty easy job.
I drilled lubrication holes in the valves and sprayed in some silicon.
This works really good. They pull very smoothly now.
Valve lubrication making lube holes for silicon grease
I also added R23 insulation around the frame perimeter and across the entire bottom before putting the coroplast back up. Hopefully it will help prevent freezing tanks and vlaves.
First picture is the valve with the crushed seal.
The others is the added insulation.