Given its location under the dash and that I've never seen a lighter plug powered by the house batteries, I'd say it's the chassis battery. Can you turn "off" the chassis battery to check? I have a chassis battery disconnect switch inside the MH. Or, you could remove one of the chassis battery cables to prove or disprove it. I would do the negative cable to avoid any arcing.
1998 Fleetwood Flair 25Y--P30 Chassis - KD5ALG
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