Tyre wear is dependent on tyre type and usage - all-terrain tyres used at high speed on dry tarmac will have short lives but the same tyres at low speed on wet grass will last for ever.
Life of tyres also depends heavily when you replace them - those of us who replace at 3mm tread depth, for road safety reasons, obviously don't get as much life as those who go down to the legal limit of 1.6mm
Apart from the handful of enthusiasts who go "green-laning" and rallying caravanners, I'm surprised that so many 4x4s use "terrain" tyres yet spend all their life on tarmac.
My present towcar is a conventional estate but with 4wd - it's OE tyres were "all-season" specification which seem eminently more sensible for UK caravanning than "summer", "winter" or "terrain". They have more wet grip than summer or winter tyres, albeit at the expense of ultimate grip in dry and snow/ice conditions - ideal for the UK climate I'd say!
To add anecdotal brand evidence, my OE Yokohama Geolandar G900B will last to 27,000 miles as a set of 4, down to 3mm. They'll be replaced by Pirelli Scorpion STR which should last 50% longer.