When we take the motor coach we can leave them in it if we need to go anywhere that we can't take them, but with taking the car only, then my husband just stays outside to walk the babies around while I run in to get whatever is needed.
If we need to stop for a bite to eat (if just in the car) we will usually just look for a Taco Bell or In and out burger, that has some grass around it and one will go inside and get the food after we walk them around first. It all works out.