2007 Monaco Dynasty Intellitec Big Boy Battery Isolator Relay 

Corrosion on the contact points inside the relay is a very common failure mode for these relays. They run about $250 new on Amazon at the moment, but it's easy enough to clean up the contacts yourself. If you don't feel like doing this fix yourself, here's an affiliate link to purchase one:
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Many thanks to James ‪@AZExpert‬ for his (much more professional) video on how to accomplish this task.
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