Any full sized truck will be at least 18' long and close to 7' tall - Putting any type of sleeping accommodations on the truck will make it higher unless you choose a cab high canopy. In the Northwest, I don't worry as much about tunnels as I do low hanging limbs on trees. I have pruning shears with me and am not adverse to stopping and climbing up the ladder to the roof to make sure I clear a forested campground road.
My suggestion was to use the truck camper as main living space and and use an enclosed trailer as a bonus room which has multiple purposes.
Considering your wife's adversion to risk, look into a mini car like a Smart or Fit rather than an open air vehicle. This will also help you when you need to access a congested city. BTW: My wife is not a big risk taker either, but feels in control when she is the one driving off-road.