Yes the problem could be something as simple as a blown fuse. The 12 volt auto type fuses should be located along with your 120 volt circuit breaker panel in the electrical compartment of your motorhome, which is sometimes located in one of your outside storage compartments.
If you do not find a blown fuse the problem could be a bad monitor panel or loose wire connection to the monitor panel. Since you are not getting any tank level readings for any of your holding tanks I do not think the problem is being caused by bad holding tank probes.
I hope this helps.
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