If the tank is under the camper, it will be exposed under the camper. Like this 2004 unit we had. And any leak would go on the ground
If the tank is inside, it "might" be under the jack knife sofa. They pivot up. Lift up the seat cushion. They also fold down into a bed. If you want to service something under there, you can take the seat cushion off. Or the entire sofa.
Here is how I did it. Lift up cushion. have 9/16" socket and ratchet handy. Take these 2 nuts off each side and the entire cushion comes off.
Now take up the luan board and under it may be the tank if it is inside. In my case it was the furnace and the HW heater
Since you did not say you yet turned on the pump, then you have a gravity leak. Maybe cracked tank if inside, maybe but more probable is a cracked pipe or pipe not connected. You are going to have to go hunting.
Maybe try this before the next wet test, find on the outside of the camper the city water hookup. It looks like a female hose coupling. Screw in a fitting to allow compressed air to go in. Use about 20 to 30 psi and leak check with air first.
The city water inlet looks like this
Hope this helps and good luck
2004 Sunline T310SR
2005 Ford F350 Lariat, V10 with 4.10
CC, Short Bed. Integrated Brake Controller
Reese HP trunnion bar hitch with HP DC