First choice: Top off while unhitched. This gives me the option to shop for best price.
Second choice: Flying J. Use the car entrance (for all of the reasons listed above). There are usually 2 RV lanes easily accessible from the entrance ramp, with pumps on either side of the lane. The RV lanes are designed for big rigs to easily move in and out. Pet Peeves: 1)Non-RVers who use the RV lanes because they didn't want to wait 2 minutes in one of the six lanes for cars 2)RVers who leave their rig in the RV lane and go in to shop.
Third choice: Anything it looks like I can get in and out of easily. Since my fill is on the driver side, I look for the far right lane. For me, access is more important than saving 3 cents/gallon.
If you are a Good Sam member, I highly recommend the Flying J card.