With our previous 35' mh and now with our 43' mh, I ALWAYS use the truck stops and truck lanes. I also have the P/FJ charge card and can pay at the pump and be in and gone in just a few minutes. As far as the "dedicated" rv lanes out in front, those are usually being used and as previously mentioned, will have a vehicle (not necessarily a rv) camped out there while the owner is inside shopping or using the restroom. Normally, you will almost always find a truck lane open or just one vehicle to wait. Truckers are very good about pulling forward if they have to go in and pay so that you can pull in and start your fill up. If you have to go in to pay, be sure to follow suit and move forward for the next guy.
Be aware that typically, the truck stops will advertise (big lighted bill board) the cost of diesel fuel at the lower commercial rate that they charge the truckers. Private coaches like ours will be levied an additional tax per gallon so the price will always be higher. Having the P/FJ charge card can give you a few cents discount that helps.
Hope this helps
Ron