Hi Kevin, Just a couple of points. I run a 04 w22 with the 8.1 & a 5 speed. pushing a triple e 36' 21000 lbs & towing a 2010 equinox awd. I run at 60mph on cruse rpm at 2050. This week on NV hwy #93 on the long flat desert the dash showed 9.9 ave mpg.
I think you should check the hitch on the rig it is likely rated at 5000 max. The Insurance co. will jump all over this if you have an incident.
I would also suggest you use a top grade synthetic oil, as this engine runs very hot.( My preference is amzoil).
Workhorse has an add on air intake to push more Air past the manafolds to help with the cooling.