We have always used the truck lanes with no problem. It is sometimes an annoyance to have to go inside and leave a card behind(we have one card that we use for this purpose only). After filling, I then pull up and go in to pay.
The discount at Flying J is good at any lane, RV, regular or truck. No problem whatsoever. I just got the new card and they are planning to add pump start at the truck lane to it. Apparently it has pump start at the RV lane already. We didn't get our full discount at a Flying J RV lane. We needed propane so we decided to stop and use the RV lane, besides small hoses, we used 2 pumps(2 tanks-they wern't a master/slave set up) and they only applied the discount to one pump. I didn't catch it at the time so it is our loss.
My biggest frustration is that each truck stop does thing differently. We just stopped at one where I had to pre-pay(tell them how many $$ I wanted, would not pump more than I paid for and this was during the day on the truck side) before I could even pump!!! This was a Flying J. Wierd!!! I think it was in Knoxville area. Most of the time we just leave our card behind at the truck desk. We have dual tanks and find filling up on the truck side is much faster, usually high speed pumps.
A book we use is Fueler's Friend. It has the diesel stations both by Interstate and town. Have bought it for years and use it extensively.