Towing 44' 21K Fifth Wheel

We just recently purchased a 44' Voltage fifth wheel with a 21K GVW. We will be traveling all over the US and want something that will adequately and safely take me through the mountains. Yes, I have several years of over the road driving experience. I have been looking for a decent truck to pull it with and have decided on a single axle tractor with a sleeper.

So here's the problem: There's not a lot of single axle trucks with sleepers out there and just about all the single axle trucks are day cabs. I have heard of shops converting trucks to single axles. Just wondering what other's suggestions are out there... please.

 

I was hoping that by dropping an axle I would be able to shorten the frame, or at least that's what I'm seeing others do. As far as getting around town, we will be getting a street legal dune buggy that will fit into the garage area of the toy hauler.