The white grainy dust was the Styrofoam core material that some small ants were carving out to improve their living space.
When we bought the camper it had water damage in the roof and the floor. I suspect that the ants were there from the beginning. We would occasionally see a few small ants here and there, but not enough to cause alarm. I should have been alarmed.
I had a bad section of headliner in the corner inside the high shelf above the 23b front bed. I removed the water damaged fiber board. There was no Styrofoam sandwich. I believe that the ants had cored most of that out, but moved on because I never found the nest.
I learned that it was a mature nest when I came out to do further work on the trailer and found it full of larger flying ants. I believe that they are acrobat ants. They were not the typical larger carpenter ants. The flying ants indicate a mature nest at least a year old.
Anyway, I drilled some access holes to spray insecticide into the cavities and used a Marine Deck plate as a hole cover to open up and gain access to the rafter area for inspection and spraying. After bombing the camper, so far it seems we have won the war.
My leak repairs last year were successful. There was no wet wood above the headliner. I've learned that for nesting the ants will continue to chew out Styrofoam and water damaged/degraded wood even when it is no longer wet.
If you see white grainy dust and even just a few ants it is best to assume that you have a nest.