On a short bed truck, which I assume you have based on the fact you are looking at the 855S, an even longer extension than what I use will be required, (I have a loing bed truck and camper) hence the reason the dealer is saying 48". Definitely no way I'd use that on an OEM hitch, on a shortbed truck.
Based on the floor length of the 855S, which says it to be 8'11", and putting that on your 6'6" shortbed truck, 48" of hitch extension seems a bit excessive based on my experience with the set up I am using:
9'0" floor length of my Arctic Fox 811, which I have spaced back from the front of the truck bed about 3-4".
24" Supertruss extension. Hauling my small trailer, I could easily use a 21" extension, which is the shortest one made.
