Here's a thought: just put on a pair of black boxer shorts before you leave the house....drive to destination...get out of truck (carefully, even with the boxers on but NOT as carefully as you would if you were not wearing them)...proceed to men's room...remove boxers...carry on. Reverse procedure when it's time to leave.

No matter what you've heard, it's okay to wear something underneath and if it's going to be an issue, don't put yourself in the place where you find yourself actively worrying about stuff like this...just take precautions. If you feel like roughing it, take your chances. This is about enjoying wearing the kilt, not stressing yourself out.

Best

AA