It's one reason I dig using mtb's as touring bikes. You can fit skinnies or fatties without worry.
I have two sets of tires and rims for my current ride and swap them out depending. One set is 1.5" minimal tread and goes fast on pavement but when riding on gravel dike systems I worry about pinch flats or cuts. They also run at higher psi and tend to throw rocks like a pistol every so often. The other set are thicker, a little lower psi and handle a lot better in gravel yet still run well on pavement because of a pronounced center bead.
As to a road bike? I don't know. You are stuck trying to squeeze the biggest tire that will fit within the frame and that's not very big. And depending on the trail, risk throwing your rim out of true more than a beefy 26" 36 spoke rim.
Perhaps a good compromise in the short term would be to buy a cheap strong older mtb and throw a rear rack and some non knobby midsize tires on - total cost about... 100-200. Then you would have a low geared touring bike that could handle gravel or road and would do until the better touring bike came along.
Last edited by Happy Feet; 01-29-16 at 07:28 PM.