You may need to do some estimating.
If you measure back from the hitch to the body, then deduct this from the shipping length would be the most accurate.
I think the internal length is the visible floor. So just adding front and back wall thickness could be dodgy.
If you look, the difference between the shipping length and internal length is 5’5”, considerably longer then the A frame and hitch.
Covers have a bit of leeway, you don’t want one too small, and one a little too big may be useful for a foture van.
John