Second Mattn suggestions, as to what budget, how old a vehicle can you tolerate etc.
I'd add two comments. As one who has towed the same 'van (a 2-berth Bailey) with a petrol Subaru Forester, and the current Hyundai diesel tug (see below), the difference was largely in the number of ratios in the auto box. The 4-speed Forester 'hunted' between top and 2nd wildly through hill work, and was never happy on the open road: any slight rise and down she'd go, up went the revs and the tank went Down. The 6-speed Santa Fe has a far superior box, and it hasn't changed much since its introduction in 2010 except to add more ratios in the very latest incarnation. It seems to be long-lived mechanically: nine years is enough to bring out any horror stories. It towed Baby Bailey over Takaka Hill at an even 2000 rpm - right in the torque band the whole way, and at a nice, consistent road speed. I had the auto oil changed at 200K and the specialist auto transmission firm which did the job commented that it was clean, had never cooked, and that Korean/Japanese 'boxes run cooler than Euro ones (80-90 versus 110-120 degrees Celsius) and last longer thereby)
The other aspect re diesel is that the running-in period is crucial. The fuel is a lubricant, which implies a much longer time for rings, cylinder walls etc to bed in. So a vehicle which spends this period knocking around town, with short trips, frequent starts and stops, and low engine speeds and average temperature, is likely not to last as long as a country leased vehicle (mine was, it put up 164K in just under 4 years) where the engine is revving, being worked for long periods, and being started and stopped much less frequently. Diesels like consistent loads, even revs (that multispeed box, again), and long hauls.
It's also worth noting that town work, towing, high speed and frequent stops and starts are classified in my Hyundai manual as 'severe duty' and this halves the service intervals. I do my own oil and filter changes, and the engine oil change interval is 7500km for this duty. So the cost of filters, an ACEA C3 or C4 oil at $10-15/l and 6.7l needed, can mount up if one is not forewarned. It's around $120/change or 16c/km. RUC is currently 6.8c/km, and at an average 10l/100k fuel usage, at say $1.20/l truck-stop priced, that's 12c/km for diesel. So there's 34.8c/km in oil/fuel/RUC alone.
More grist for the mill....
Wayne and Madeleine
Baby Bailey 400/2 'van, Hyundai Santa Fe (CM) CRDi tug.