Have you pulled the unit from the wall to check the connectors on the back and side? I found this wiring diagram and it doesn't mention any HDMI so you might be stuck with analog. Is yours wired like the attachment with the red/white RCA cables from the TV sound Output going to the marked Input? Just checking...
I assume you have a manual for the Furrion. If not you can get it
HERE
As an alternative you might consider switching to the DV3300. Furrion's AV stuff isn't the best quality to begin with but it should fit in the same hole as the 3100 and allow you to use HDMI.
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Jeff & Sandi (and Teddy - 7lb Schnorkie)
2018 Montana High Country 305RL
2015 RAM 3500 Crew Cab 4x4 DRW
Demco Recon Hitch on RAM Puck Ball