number one tip - don't obsess over saving small amounts per gallon. even at 100 gallons, saving 10 cents/gal is only $10 versus how much frustration you could deal with getting into that cheaper station that's super busy.
number two avoid high density retail areas ... if the exit sign is advertising 6+ gas stations, 6+ fast food outlets and who knows what all at one exit, chances are it's all crammed together on a busy street. look for exits that have a truck-stop type gas station, often times they'll have a huge (tall) sign you can see well before the exit.
30 ft trailer and a 18-20 ft tow sounds big, but just take it easy, be patient and go slow - you can fit into just about any gas station as long as there's clearance. that station has to be able to accept the huge fuel hauler that delivers fuel, so it can accept your RV combination too.
regarding "just truck stops" I've never met a retail station that didn't sell gasoline. there are some fleet-based ones out there (where it's just a fuel island but no store/building), but those are pretty rare. just about any truck stop you pull into will sell gasoline. there's plenty of gasoline powered delivery vehicles out there, mostly smaller bread trucks / box trucks.