I cannot provide a cite, but I believe several state AG's have said meal and rest stops are ok for coverage under FOPA. Anything else is verboten and if you do you had better be in total compiance with the laws of the state you are in.
Since there are quite a few states that are too big to drive across without a bathroom break, a common sense approach would be to assume that those kind of stops are legal, and I have seen that asserted on this and other gun sites. I wouldn't want the attorney's bill for a test case, but I don't see how the law could be meaningful any other way.
If all firearms are stored out of sight and secured as mandated by the federal law, it seems to me that a search warrant would be required for a LEO to find them and cite you, anyway.
My field is far away from the law, however, so you need a more authoritative answer than an online forum.
