See my Sig. I basically have the setup you're looking for. On an older truck with a big trailer I consider the following to be must haves:
Diesel with a tuner. Mpg towing average about 8.
Exhaust brake.
6.0L transmission cooler.
Dually
I live in Oregon and do lots of mountain driving. This setup when driven carefully does just fine. Truck only cost $10k.
Also, I bought my truck with 200k miles from a gentleman that had taken very good care of it. I tend to not be afraid of high mileage vehicles.
Given all that, a 40' trailer is a PITA to navigate around heavily wooded tight campgrounds, but I do love it once it is set up.
