I would expect it to be in the Front Distribution Panel, the back box inside the compartment under the driver's left foot, accessed from outside the coach. I can't say for sure on your 2003. I downloaded your coach's owner's manual (link here
), and it didn't give a drawing of the FDP. Some of the manuals do.
Here's a link
to a photo of where it is on our HR's FDP. Trace the blue wires up and to the left and you'll end at the BIRD (Bi-directional Isolator Relay Delay) module, which controls the automatic activation features of the solenoid. The AUX BOOST or BATT BOOST switch on the dash controls the solenoid manually. You could have a faulty solenoid, BIRD or switch. Each of these is easy enough to test. In my case, when I bought the coach the BIRD module was broken. I replaced it. I've also bought a spare BIRD, and a spare solenoid.