I had something similar happen to my 5er a couple months ago. Lights got dimmer and dimmer and dimmer. Everything checked out just fine. Couldn't figure it out. What I failed to do after we returned home from the week-end trip when this happened was .... well.... (do I really have to admit this ... well OK, if it will help you) ... I forgot to flip the breaker on at my RV electric box!
The residential refrigerator was running of the inverter for quite a while, drawing down the battery, making lights go dim after a few hours. DUH!
Check to make sure you flipped the breaker back on at your power pole! It might be just that simple.