Unfortunately, I do not know your factory system and how it is designed to run. It is too bad it is in a difficult place to access. Here are the things I would do to begin with.
1. Make sure your batteries are good and fully connected, that is clean the terminals, both positive and negative. Do the lights and all 12 volt
things work as they should when the rig is NOT plugged into 120 volt power? Make sure of that first.
2. Look for any fuses in the solar system lines. Check their continuity with an ohm meter to be sure they are good. Occasionally they will look OK but in fact have a hairline crack and won't pass power.
3. Look at the charge controller for fuses and check them.
4. Next check the panel output. If you can access the line which comes from the solar panels, and can remove it from the controller, remove it (mark it so you can re install it the way it was) and set your multi meter on DC volts. In sunshine, measure the Voltage across the line. It will probably be in the range of 17 to 18 volts. Then set the meter to Amps and measure the current flow from the panels directly across the meter (that is short the panels through the meter). You will probably have to change the lead location on the meter to measure Amps. This will not damage the panels at all, and will tell you if they are putting out power. You will probably measure over 6 amps and under 20 amps. Be sure your meter will measure 20 amps, some will only do 10 amps.
5. If it passes the tests in 4, above and measures about what I mention on volts and amps, and all your fuses are ok, and battery connections are good, I would next suspect the charge controller itself, as it is the only thing between the panels and the batteries.
Diagnosis of electrical issues by forum posts is an inexact science at best, but those instructions may get you started.