I wouldn't do it in a large rig personally, there's some tight switchbacks that would be very hard to make with a large rig, I can hardly keep my Silverado between the lines on them, you may have to make some multi-point turns to get around them with a long rig. The tunnels are pretty tight, and would be beyond my comfort level for an RV, but I know people have done it and some tour buses do it regularly too (though they may be smaller than some RV rigs, so KNOW your measurements!), They post vehicle height/width restrictions at either end of the iron mountain road, so as long as you're within your comfort margin, go for it ;)
Sept 2014 video from one of the iron mountain tunnels in our Jeep
