yes typically water tank. as noted top generally screwed on, buried in carpet. Not room for much else in the box- though good to access for running wires across camper etc. On ours the gas lines, bundles of 12v & 11Ov wiring crosses thru tank box gas. Any wiring Ive add I used box to cross camper. Some you can access tank fill & drain connections from top-or at least see. Not a lot a structure- frame on ours is a few pine/fir 1x1's-prying would not be advised.
I didnt rescrew ours, used some angle brackets on wall-top slips under and used screw hook & eyes to hold front. But I added an access port in tank so wanted easier access for refilling tank on occasion.
