Really, there is no advantage to using trusses to span the 44', and leaving 10' of it open. If you are already putting up a new building, the only real extra cost associated to enclosing all 34x44' is an overhead door. A dividing wall to separate the 2 spaces could be as simple as a 2x4 stud wall sheathed in OSB. If you would prefer the RV carport area open, then the lean to construction would likely be a more cost effective option. IMO, you're spending the money already to build the shop, whats a few more thousand for the extra 10' of potential future shop space. Also, keep in mind that in the future, if you want the larger shop, you can always add the lean to onto the 34x44 shop...it never ends haha.