New member, new rv'er forums are how I survive.

In southern California I added my RV plate number and my toad was already on the pass so i just tow on my toll roads and they bill me accordingly.

Various trailers I tow just get billed as extra axles. I don't worry about getting charged, they always do....
As far as ag stations, once I was asked if i had any fruit or vegetables I just say no. Makes life easier.
Commercial vehicles only in weigh stations UNLESS it states rental trucks, like entering Las Vegas area.