x2 on most of this advice.
It is trial and error and learning by experience where you can fit your rig.
We've found it is usually when we are in urban areas that we have problems. They put gas stations on very small lots (probably due to the price of land) with no manuevering room.
When you are out in rural areas, there is often much more room to enter and exit.
Just make sure you've got enough gas left in the tank to pass one or two stations to find the one that can accomodate you.
And I am very pleased that your suspicions about people getting snarky with you didn't happen. It seems like all the responses have been kind and helpful. :)