How long is too long to travel In a fifth wheel.

At 38 ft you are already in diesel dual rear axle country. So you may be looking at a new truck too.

I looked at this before we bought ours and it seemed that over 35 ft was when you start seeing a lot more problems with camping in State and National parkland. And even at 35 ft there are a lot of restrictions. There are only a handful of sites in Yosemite Valley that will take a 35 ft RV for example. If you plan on sticking to commercial parks then you won't have much of a problem.