I assume part of the problem is how the tank looks. If it's all rusted up, that flags the filling station. If it looks great, they just fill it. The same generalization can be applied to those propane tanks used for grills. They need to be painted and looking maintained or the fillers won't touch them.
Our tank actually looks pretty good for something with 17 years and 102,000 miles on it

; just a few specks of chipped paint with rust in them on the side that gets hit with splash from the rear tires (because I'm too lazy to add the splash guard like you did

). Certainly nothing that would weaken or otherwise compromise the integrity of the tank. Portable tanks (DOT tanks), whether they look nice or not, ARE supposed to require re-certification every 12 years; wondering if anyone ever actually does that?