I have the 28BHBE (the Jay Flight cousin to your trailer) and mine is under the sink. Pretty easy to access. If it is back by the water heater, you can access that from the bottom bunk. Take the mattress out, and there should be a flip-up panel for access to the WH (look for a finger hole, this is how you'll access your winterizing bypass valves), if the pump is back there, that's how you'll get to it.
I doubt it's back under the bunk though, because it should be as close to the fresh water tanks as possible, which should be just fore of the front axle on the curb-side of the unit.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!