On the opposite extreme, I use my Fiat Fiorino van to tow my 6m rib around Pembrokeshire
Tis only a 1.7 TD, and 2-wheel drive, but it seems to get the boat up the steepest of slipways without too much clutch. Once going at about 3mph, it has enough power to pull itself up pretty steep gradiants in 1st gear. I have so far only got it stuck on a slip once when the trailer got stuck on a rock - that required another car in front with tow rope to add to the pulling power. When the slip is really bad, a JCB loadall comes in quite handy - it takes quite a lot to get that stuck
As for motorway, I won't be taking the boat behind the van on that kind of road. There isn't quite enough 'go' in it, and I reckon I'll be down to 30mph on most slight hills whilst watching the large artics go flying past.....