Do you get water from faucets and/or toilet when not connected to an external supply line? If so, do you hear a pump running? If so, open a faucet, and follow the noise -- it will surely lead you to your pump. I would guess that your access panel will be on or next to a lav, sink, toilet, or shower, and the panel will be on the INSIDE of the rig.
By the way, I saw a question about whether it could be "gravity fed". The only way that would be physically possible is if the supply tank is located ABOVE the highest faucet. Unless your tank is on the ROOF, that seems unlikely.
2006 Wildwood LE 28FKSS - 2015-
Read my weekly Computers & Technolgy Q&A feature It's Geek To Me