This definitely sounds like a tripped Automatic Transfer Switch (ATS). There was a period when what Montana installed would fail. You may not have a generator, but you are probably "generator prepped". This prep includes the ATS. It is a simple and inexpensive device to bypass. Even if you are not electrically incloned, a mobile guy can do it.
Don't walk away from your RV for what is an easy and inexpensive repair.
Some previous discussions on this problem:
https://www.montanaowners.com/forums...ad.php?t=84425
https://www.montanaowners.com/forums...ad.php?t=84727
https://www.montanaowners.com/forums...ad.php?t=83044