You can go through the truck pumps, but several of the Flying J's, Pilots and others have lanes specifically dedicated to RVs.
The Truck pumps usually have a much higher volume that they dispense. So if you're refueling, it's very easy to cause a major spill if you don't monitor the pump and shut it off when it's getting close to full. I can hear the change in tone when mine is getting full, and shut it off accordingly.
As far as etiquette, wait your turn in line at any of the pumps. Fill up promptly and exit the lane as soon as you're full. If you need to go in to pay, you can pull forward far enough for the truck behind you to get to the pump so they can start their fill up. Do not block the lanes needlessly.
As far as paying at the pump, each station has different requirements. I signed up for the Flying J/Pilot credit card, and I can pay at the pump at any of their locations. In addition, you get $.08 off of the CASH price at the pump when you use their card. Generally, their limit is $1000/month if you pay the bill via normal methods, or $2000/month if you sign up for their automatic debit.
Jim