I am a civil engineer who has experience with site design. I'm not specifically going to defend the gas station owners, but in many cases their lots are not big enough to accommodate large vehicles and trucks, unless they are specifically designed for commercial traffic (such as truck stops).
What happens is that over time, highways are widened, which requires strips of land to be acquired along the frontage. This eventually reduces the land that the gas station has to work with, forcing modifications where pumps and/or buildings have to be relocated. The gas stations also have to meet township codes which require certain offsets to property lines, so this becomes a challenge as the lot shrinks.
Newer gas stations built on large lots don't really have an excuse, but most of the older stations do have challenges when it comes to accommodating larger vehicles. In many cases the lot is just not big enough to properly fit the pumps, awnings, buildings, etc that RV'ers can negotiate easily.
Also, on state highways, most DOTs regulate the access to the lot, especially if the property is at a corner of an intersection. So the owner can only have a certain number of driveways, and they have to be a certain distance from the intersection, at a certain angle and width. This is done to control the flow of traffic and reduce the number of conflicting turning movements. It dictates where a vehicle can enter and exit the gas station, and many times not to the advantage of someone driving a trailer-type rig.
