150K is a lot of miles on a RV. Our 03 now has 57K and it only had 33K when we bought it in 2007. I was thinking we were putting a lot of miles on ours. The rig you are looking at has three times as much and a year newer.
Do you have any maintenance records on the rig? At the least I would have a trusted mechanic give it complete going over. The 6.0L / 460 LE tranny are a great drive train but that would be an expensive replacement if they fail. The nice thing about the T/L is their light weight ( ours is 9K ) so the drivetrain should not have been abused like a big class C with the same set up.
If it way below book and I was going to stay local ( a few states in any direction ) it might be a great deal. I would really wonder about a cross country trip with that high of mileage. I would definately look into emergency break down / towing service if I bought it.