system sounds like the one in my sister's Compact Jr. She kept the two separated systems, and we put LED bulbs in the 12V fixtures so her battery
would last longer.
For reading lights she just put up a bunch of those self-contained LED "tap" fixtures.
But if you're going to add a big roof fan, you're going to need a more sophisticated system. I don't know as I'd ditch the 110V lights, as Frederick suggested, but I do think you're going to need to add a converter so that you can run 12V stuff off shore power.
However, now that I think about it a bit more, you might want to reconsider the fan. When you pop the top on a Compact, you're lifting the top all by yourself, no helper springs, no air cylinders, just your arm strength. Adding a fan might be a bit much to lift. Also, my sister's Compact had a warped roof; when it was time to try to get the vent to seal, I had to add some bows inside to push the roof up into a nice shape before I could count on getting a good seal. A couple of us here on the forum have added reinforcements to the roof; one fellow fiberglassed in aluminum tubes, I think, and I bent up some bows out of conduit and bolted them in. It's not impossible, mind you, but it's a bit more work than "cut a hole and bolt the fan into place".