Micar has it right.
I have to have an annual inspection for the dually because of the weight rating of the truck. Therefore I have to have my 5X8 enclosed trailer and my 6X10 open trailer safetied every year as well. I have seen 1/2 tons with the yellow sticker because the combined weight of the truck and the trailer exceed the magic number.
However, I don't have to do anything to the travel trailer, no inspection whatsoever.
It makes things easy if a buddy wants me to tow his trailer... No Yellow sticker... no tow.
Yellow sticker is safety inspection, no need to do the emissions test until the truck is 7 years old( or did they up that to 9 years? )
