I had my new 4x4 F350 diesel dually for 6 months as a daily driver before I went full time (now it is obviously my DD). During that time I drove 60 miles/day just back and forth to work - so about 10-12k during that time period including other travel.
I traded in my 6 year old Honda with 180,000 miles on it (still ran perfectly).
In your situation I would get a dedicated older truck and use it only for towing. You will drive 100-125,000 miles in the next 5 years - sounds like mostly highway. Why not put all those miles on a vehicle that is much cheaper to run and maintain and that you already own ? Just as an example, you may very well wear out 2 sets of tires by the 125k mark on a truck. But I could get 100k miles out of a set of (cheaper) tires on my Honda. Will your new truck be a 4x4 ? If so, why put 125,000 miles on a 4x4 drivetrain if you don't have to - and yes, I do know about lock out hubs.
If you decide on a 5th wheel, you may very well get a long bed truck. Also will have the hitch in the bed. Maybe an aux fuel tank/tool box. Will just be easier to drive your current vehicle as a daily driver.
Just my .02
Tim