Yep, it happens..
Last summer, we were headed out to camp for long weekend, taking our 4yo grand daugher with us.. of course she had to go potty between little towns on narrow 2 lane with plenty of traffic.. she could not hold it.. so pulled over as much as I could on shoulder and let her use the TT bathroom.. had to make several more stops before we arrived at camp (only 120 miles) as using TT bathroom was fun.
Note, using the trailer with slide in was not on our have to haves when we were trailer shopping so did not even think of it. Our trailer is one that you really can't use with the slide in, but we can get into front bedroom through front door, and if we leave bathroom door open, we can get into bathroom (the slide blocks it from opening if we leave it closed).. so the important aspect of using toilet if needed when traveling works out...