You just go.
You should buy gas at any station that looks roomy enough to you, once you are down to about a 1/2 tank, so you are not in panic mode about finding a place when it is almost empty. Most C's have nearly 400 mile range, they fit into gas stations easily in rural areas (don't try to fill in cities and suburbs) and there are no places where the stations are more than 400 miles apart.
Walmarts are where you find them. Small towns have other stores with emergency supplies, and RV dealers are more frequent along the highways than you would expect, you just don't see much of this from the Interstate Highway system, that was intentional in the planning and design.
Very narrow roads? You have to get off the highways, to find the narrow roads, as the highway system is designed for 8.5 foot wide trucks, up to 65 feet long and 80,000 pounds. Your C is a lot smaller.
Where you find more narrow roads will be on city streets, particularly residential areas, or on secondary and tertiary roads in rural areas. A RV doesn't belong on city streets;you just don't go there unless you know the route well.
On back roads in rural areas, you will be on slow speed roads where there is a protocol for two vehicles sharing a space less than 16-18 feet wide; most of the time these have no traffic. When you see oncoming, it is often best to pull over and stop, the locals know more about what they are doing.
Otherwise most narrow road situations will be construction zones, where you have to slow down to thread your 8.5 wide RV though a spot 11 to 12 foot wide. Any narrower, there will be warnings for wide vehicles to take detours; pay attention to that. You will build "staying in lane" skills quickly driving a RV. Often drivers can't keep a 5 1/2 foot wide vehicle within a 14 foot wide lane, because they've never had to concentrate that much on the task. Drive a RV for a while, you'll outgrow that lack of concentration quickly, all it takes is making driving your primary task, instead of something you do while eating, texting, or brushing out your hair.
If you've not driven in mountains, you'll need to learn to control your speeds downhill with minimal braking, which means using engine braking to maintain the speeds needed to make it around the curves, rather than running up to 70-80 mph and them trying to slow to 35-45 to negotiate the curves. Speed limits in mountainous areas are set to maximum safe speeds for the whole section of road, pay attention to that.
P.S. Walmart is not a particularly good place to buy fuel, whatever your discount/rewards program, if in a RV. Many of the discount store and supermarket fuel sales points are way too crowded for negotiating large vehicles. I like the rural gas stations that cater to farmers and ranchers, who are almost always towing something when they pull in. The pump layouts are designed to take care of that, you'll learn to recognize them. Clue: pumps parallel to the highway, wide entrances with no sharp turns or change in grade.
