So now safely in Canada and think we've found a good option:
2006 Ford f250 sd xlt supercab short box 4wd
I believe it has these specs:
Curb Weight 6,207 lbs
Gross Vehicle Weight Rating 9,200 lbs
Max Payload 2,993 lbs
Max Tow: 8200
The 5th wheel I'm look at is a Jayco Eagle 281 RLS
Cargo capacity 2078lbs (tanks full)
GVWR: 10500
GAWR:5000
By my calculations we will be good for this set-up - but without a huge margin. There is an option for an older f350 but then we start getting into a bit rusty, a bit too old, wrong side of 200km.
Thoughts very welcome!