Camper lengths

Question: I bought a 12'x16' tarp as a temporary cover while it's out of storage (just a couple weeks). It doesn't have leaks but I don't want any to form, and want to keep the tree debris off the roof, and dont have a proper cover yet. 

I remember all the specs saying my camper should be 11' long. But the tarp didn't fully cover it at the 16' length. Are (truck bed) campers measured differently than full end to end length? I feel like I'm missing something here. 

It is a 1994 Elkhorn 10W

TIA