We are considering the Missouri River this June. I take a folding luggage cart that transports 2) 6 gallon gas cans with ease. We often get gas at river towns at regular gas stations and convenience stores that are a short walk from the river. They are easy to find using Google map search.
I just did a quick check and found gas within 3 blocks of the river at the following towns.
Brunswick, Mo
Glasgow, Mo
Boonville, Mo
Leavenworth, Ks
Atchison, Ks
It may take a couple of trips to get enough gas at only 12 gallons per trip, but at our age, we need a bit more exercise anyway.
Even places where the gas may be too far to walk, gas can still be had.
I have had folks at boat ramps offer rides.
Many small towns have a small independent cab service, they will do a gas station trip for a reasonable fee.
Stop at any town with a Domino's Pizza, and as long as it is not lunch or dinner time when they are busy, the delivery driver will haul gas for about the same tip as they get to deliver pizza.
Also, one of the most eager and underpaid people that are sitting around with not much to do most days are flight instructors. Any town with a small flight school has a starving flight instructor with a car that can be hired for a gas run. On 2 occasions I have hired flight instructors to drive upriver and drop me and the boat at a launch point and shuttle my truck back downriver.
Last year on the Ohio River a man was mowing his grass in a small town and watched as I walked by carrying a gas can, He sat the gas can on the mower deck and let me ride straddling the front hood, and proudly drove me back to the landing where Sharon and Bixby's Cub were waiting.
Never tried it, but if you had 2 or 3 boats and could purchase 100+ gallons, it may be possible have a fuel service meet you at a landing, boat ramp or bridge. These are the guys that bring fuel to construction companies and farmers
I like the areas without marinas, No marinas, no cigarette boats, no jet skis. The more miles it is between traditional gas stops, the more you will have the river all to your self.