OK, I thought you were measuringat a light fixture, not the switch.
But, the switch is interrupting the + 12 VDC side to the lights. I assume that 3.8 VDC to ground (and to the light side of the switch) should be 12 VDC. The -.25 VDC on the light side is just stray induced voltage, and I assume that is measured with the switch open.
So probably not a ground issue but a + 12 VDC issue - a corroded or loose connection somewhere. Time to back trace, that wire going to the switch. I would assume all lighting is on the same fuse so if the other side is working then there's probably a crimp splice somewhere that is the problem.
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad