In many cases its simply a breaker issue.
I find at RV shows, dealers will hit the disconnect expecting the lights to go out, but they are plugged in so they don't. So then they go inside and flip the breakers (lights go out). The next day when they come, they hit the disconnect switch and voila...they have lights, so they move on. However they are now only running on battery power. I see the lights go dim. Check the plug, then check the breaker. Might not be the problem...but its the first place I would look.
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