If your older potty trained child falls asleep and wets while they sleep then yes you should make the older potty trained child wear a diaper or at least a pull up in the vehicle on a road trip.
My 6 year old son who is potty trained always wears a diaper on road trips because he sometimes wets his pants while we wait to find a toilet and sometimes he falls asleep and wets his pants so the diaper is put on him to keep his pants dry and the seat dry.
He wears the Gentle Steps size 7 diapers which do a great Job at holding in pee.
I tell him it's okay if he needs to use the diaper and tell him not to hold it because it's not healthy to hold in pee.
To change a diaper in the car place a towel or changing mat on the seat such as the front seat or back seat.
Then lay the baby or child on the towel or changing mat and then change their diaper as normal.
If you have a trunk that is also another place to change the baby in the car.
Things you should pack in the diaper bag on the road trip are enough diapers for the road trip, baby wipes, sippy cups, diaper rash cream, bottles, a change of clothes for your baby or toddler.
To dispose of dirty diapers on a road trip place the dirty diaper into a plastic bag such as a grocery shopping bag and then tie it up.
Then place the dirty diaper in a public trash can.
Gas stations usually have trash cans near the gas pumps where you can dispose of your child's dirty diaper.
Or at a rest area there should be trash cans to dispose of the dirty diaper.
When taking a road trip with kids in diapers you should pack enough diapers per day to last the road trip.
If your baby or child goes through 5 to 8 diapers per day then pack at least 8 diapers for the day.
Or pack 16 diapers for the day and the next day or double up and pack a few more days worth of diapers.
For example if it's a day road trip then you should pack at least 2 days worth of diapers so that you have enough to last through the day of the road trip and the day coming back.
Or pack a weeks worth of diapers to be sure you have enough.
You can also buy extra diapers on the road trip at stores such as Walmart, Dollar General, Target etc.
You'll be able to buy diapers at your destination (and you'll want to check online before your trip to find out what stores will be nearby), but it's a good idea to pack at least 10 diapers in the suitcase.
That gives you enough to get by until you can make a trip to the store.
When on a road trip diapers can be bought at any grocery store, Walmart Stores, Dillon's etc.
In just about every state you'll pass through you should be able to locate a Walmart Store.
Walmart will have diapers.
Dollar General also has diapers and they are in most locations as well.
When traveling you should bring at least a box of diapers with you to ensure you have plenty.
I always keep a box of 120 diapers in the Van we travel in so I have plenty of diapers to change my baby with.
If we're gone on a month road trip then that box will usually last the entire trip.
However if not I can stop at Walmart and get another box of diapers and have some left when I get home.
But you're going on a few days road trip then taking a few days worth of diapers with you on the road trip should be enough.
Then you can stop at Walmart or other stores to buy some more diapers if needed.
You can also have diapers shipped to a hotel or motel where you are from Walmart.com or Amazon.com