No specific advice, as my youngest is 12. And we did not travel when they were that age,
BUT When I read the advice above it reminded me, we always used to say, to all 3 of our kids, whenever they told us they didn't need to go (usually when we were taking another kid)...
"You don't need to go but you DO need to try" I must have said that line a million times when they were little, but it used to make them feel like they weren't being made to go if they didn'e have to, of course they ALWAYS tinkled EVERY TIME... But they didn't 'have to'
My 3 kids are close in age, and I really didn't have time in my life for their bladders to be on 3 different schedules.
Even now, on road trips they know they need to go when we stop for a break even if they don't "have to"