Well, if you're "parked," there's a good chance that there are restrooms.
You can carry a portable toilet. Or a bucket and kitchen trash bags. I'm an over the road trucker, and in an "emergency" I have just closed the curtains, squatted down in the back and pooped into a 13 gal kitchen trash bag. Not at all comfortable, but workable in an emergency.
Store the bag outside in the spare tire, until I get to a suitable dumpster/trash can. (And I doubt that my rare, once or twice a year "dump" compares at all to the daily disposal of diapers on the road and at home on garbage day.)