Part of the speed limitation is the 19.5" tires that come with the truck. I typically tow behind mine and keep speeds down to 65 mph or less which I have not found to be an issue. You may find that you will travel through states that consider any vehicle over 10k lbs GVWR are required to follow truck speed limits and not the higher limits for cars. This may not be enforced but may become an additional infraction if they find a reason to stop you...
Rather than having a rear deck, you could easily have covered or open storage under your cabover. An 11' deck would give you 2-3' of storage between the camper and truck cab.