Sounds to me as if you think all services disappear when you leave the Interstate Highway system.
In reality, at least for the parts of the country I've traveled, most of it between the Appalachians and the Sierra Nevada, and all of the southeast, services become more frequently available once you get off the Interstate, and onto the primary highway system. More services because these highways go through the towns the Interstate system bypassed in order to more efficiently connect big cities.
The smaller the town, however, the more likely that you, the stranger, will be watched with some suspicion. Unless you go into somebody's business (restaurant, small store) and mention what you are doing there and say maybe you would like to stay overnight; in that case, they will likely suggest a place to park, and before you get parked almost everybody in town will know why you are there and some may come by to chat. If there is law enforcement in town, they'll be watching to keep you safe rather than rousting you to learn why you are parked overnight.
East of the Mississippi and in the tier of states immediately west, towns are spaced every 10 to 20 miles, that had to do with round-trip travel time farm to market. On the plains, it might be more like 15 to 30, stretching to 50 miles in the arid parts further west lacking intensive agricultural development, and also going through the Rockies, which are just not so densely settled.
What you find on what I think you are calling "back roads" will include truck stops, gas stations, convenience stores, rest areas, picnic areas, and free or low cost campgrounds and RV parks run by county, town, or municipal governments.
In the towns will be supermarkets, laundries, restaurants, banks, hospitals, hardwares, feed stores, parks, fairgrounds, churches, inns, taverns. All of the things you expect to find in big cities or the bedroom communities sprawling around, just not as many of each thing, and often not the nationally branded franchise businesses that locate near Interstate exits.
Rather than try to find some place to pull off the highway between towns, my inclination, my practice, is to go into town, make connections with people, and make use of what is offered or what is there.