Gate valve

I x2, if it needs seals, replace the whole valve, not just parts of it.

Probably crud stuck in the valve seat. I had a baby wipe stuck in mine, not once but twice. Both times I had to open the belly and remove the valve to dislodge.
Baby wipes are now outlawed from the camper.

Sounds like the gasket has slipped into the opening.
You say it is a 4 inch. Might verify it is a 4 inch and not a three.
I would get the complete valve assembly and replace it all rather then just the gaskets.
Blade valves

Can you get to the inside of the gate to see if there is something (probably tp) sticking in the groove for the gate valve?

Ron