Oh the compromises we all have to make! Natl. park size limits vs. wanting a separate shower arrangement usually found in the bigger rvs; high ceilings (5vers) vs. getting into the more primitive campgrounds where lower height TT's have the advantage; wanting a separate office space usually means over 30ft., etc, etc.
IF this is your first RV, I'd consider a used one first because we all learn a lot from that experience. NOTE that some folks never do learn to back 5vers well. Our first used 16ft. TT was purchased because our Ford Ranger could pull it, but the next two tt's were bought because we liked the floor plan and then we went shopping for the tv that could pull them.
From the OP's wish list, I would think that a 30+ft. 5ver would be the logical choice, knowing that some NPs and State parks won't be accessable, though more and more of them are adding longer sites to their mix.