Egad, I am not so sure that is "FRESH" water tank..
Are you really sure about that?
Those type of gate valves are typically used to drain "grey" (sink and shower) or "black" (toilet) waste holding tanks..
What is the size? 1 1/2", 3"?
Typical freshwater tanks will use standard off the shelf water valves that are 1/2" pipe thread..
Grey tanks use 1 1/2" gate valve and black tanks use 3" gate valve.
Some RVs have two grey tanks and one of those all by it's self.. That is often due to the layout of the RV which may not allow one grey tank..
That valve is replaced by removing the four bolts holding it together, removing the bolts allows the two outside flanges to part from the gate valve body.
Replace the entire valve body, don't bother with buying just the seals..