Confusing Water Tank issue

[DexterW]

I have an old 1960s camper and the water tank is confusing me. When I try and fill it, drains out of a drain hole in the bottom of the camper. When I plug the drain hole, the water almost immediately seems to overflow out of the fill spout. I thought maybe it was full or something and the drain hole is overflow but I can't find what would be the real drain hole if this was the case.

The sink pump inside the camper wasn't pumping anything, so I plugged in a new one and it's not pumping anything either. I took off the new one and put the flanged part that goes into the hose into a cup of water and it didn't pump anything again.

Are both my pumps broken? What's going on with the water tank?

[Gary RV_Wizard]

Some 60's vintage RVs used a pressurized tank water system, but if it has an electric water pump, probably not. A non-pressurized tank has an overflow and a vent (sometimes the same tube does both functions) and that is required to be open for the system to work.  It also sounds like your tank is already full. Check the tube that the pump uses to pull water from the tank. It may be plugged.

Is the "sink pump" a manual one rather than electric? Some smaller RVs used that type instead of an electric pump.

[DexterW]

How can I even access the tank? To check these hoses? There doesn't seems to be anyway to actually get to where the tank is.

The sink pump seems to be exactly this one:
Valterra RP800 Chrome Rocket Hand Pump https://www.amazon.com/dp/B000BGM2XG/ref=cm_sw_r_cp_apa_YE1YxbDTSA5NE

[DexterW]

And as I was saying, shouldn't this new pump work if I put the end of it in a cup of water?

[DexterW]

Solved! The water tank was full, and the sink spigot needed like 50 aggressive pumps to get the flow going. Thanks dudes!