How hard is it to do some research before you hit the road? I just got back from an 8 day long road trip (not with the camper, and I was solo for the first half). Before I hit the road, I researched where there were Costco locations to buy gas, and how to get to them from the highway. In areas where there weren't any Costcos, I researched the morning of where the cheapest spots were to go get gas, by using the GasBuddy app.
I would just think if you're really into stopping at Walmarts, you could spend 10 minutes or less before you start going to find out where they are located along your route for the day, without needing a sign to tell you.
I do think having the little RV indicators on the signs that are already up is nice though. We tend to do some scouting out before hitting the road using the google maps satellite and street views to see what stations might be a possibility when we are traveling with the camper.