Oil changes en-route with camper on truck?

Walmart. We tend to put on a lot of miles with friends and family scattered around so we have done 8-10 changes while traveling. We call ahead to check, only a couple have said no and one in Prescott wanted $50. Every other Walmart has done it for $20 except one that charged about $30. We provide our own oil and filter. If they carried what I wanted I would buy from them as a courtesy. Of all the ones we called only one older store did not have a high door and pit to drive over. One note though, the book says three gallons and that’s what they want to put in our truck. Our truck does not drain oil fast so unless I inform them and watch them they have a tendency to overfill.