You know it's would be a big help if we had an idea of the year, model, floorplan, chassis, etc of the RV you are asking about.
I've never heard of any "feature" to defeats cruise control when the headlights are on. I know this isn't something I've ever encountered myself. But maybe that's just luck?
I did a simple Google search and found that others that reported a similar thing all found it to be a problem with their vehicle, not a standard feature of the manufacturer.
One such result was on a Ford and the problem turned out to be a faulty light bulb in a turn signal/parking/brake light cluster. Here's his response on the fix:
Quote:
Just for future reference, I gave up, went around and changed every exterior bulb and the problem cleared up.
I found that Ford designed the ECU to disable the cruise if it detected any perceived defect with any brake or tail bulb and even though they all worked one must have had a high resistance or whatever and now new bulbs have cured the problem.
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Others found broken grounds.
So, again, it might help to know what engine, RV, etc you are talking about. Hopefully, something simple.