Bill,
If you are going to mount the light(s) in a new wood panel, a small hole in the vinyl head liner shouldn't be a problem as the wood will keep it from running, etc. As Lee mentioned, running it above the ceiling insulation would be preferable but maybe not possible. If your coach is constructed like the pre-Monaco units, the headliner has a foam backing that is glued to the ceiling panel so you may have a challenge in running it. I'm assuming you are going to use the switch for the dinette lights to supply 12V. As long as you use LEDs in the new fixture, the amperage load should not be an issue and 18awg should be fine for power and gnd.
Steve