It will be behind the water heater, back side., and probably behind some removable panel in the camper, or under the bed. Now are you sure the water heater is full? On most trailers, you have several valves to turn in order to get out of by-pass mode. If you search for water heater on here, you will find instructions, and pictures, or different set-ups, and valve arrangements.
By the way, welcome to the forums.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT