I know the situation you are speaking of and you are correct in the fact it is due to a void of insulation. You should be able to spray the cap with a foam insulation through a few opening; say 3 or 4 into the bottom of the cap. If there are no opening you could drill and access hole and then make a cover. Another way might be through the clearance lights but is a bit trickier to get it where you want it. Good luck and keep us posted.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7