Could you possibly take a photo of the roof of your truck showing the relationship between the roof antenna and your camper?
I can see how the radio signals could be attenuated if they had to pass through your camper to get to the antenna, but I'd think that signals coming over the hood of the truck would be fine. Can you try an experiment of trying the radio/app with the truck pointing in different directions?
I "think" that it may not be the antenna that is the issue - but the above tests will prove that hypothesis.