on our old popups, the water tank was under one of the dinette benches.
it didn't have a water pump. most older popups had a hand pump water spigot.
you could take it to a local popup dealer and see if they could help.
i'd bet yours never had a electric water pump but doesn't mean you can't install one.
what kind of faucet does it have? can you post some pics of your sink?
ours had a single faucet for city water, no hot, and the hand pump for tank water.
and Zoe the Wonder Dog(R.I.P.)
'07 Roo 23SS pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC