I think I would stick to the 4.1's. Since I wrote the original post, I made up a table that does not lend itself to be published on the forum which I've always had on the VMN web site
See https://www.vehiclemods.net.au/page/hil ... 2005-2015/
Also there is now a nice complete package for 4.1 gears available which we order in on request (allow about 2 weeks).https://www.vehiclemods.net.au/toyota-h ... regear-kit
We also supply the Harrop Elockers (You need the 3.73 and down) for the gear set above.https://www.vehiclemods.net.au/spare-pa ... tion-aids/
With 3.9's there is no easy package and if you are not careful you'll end up buying gears from Toyota.
As far as real world experience, I've done a lot ot towing for a 1800kg caravan on 265/75 R16's and 4.1 gears. They are similar size to your proposed size.
Just drop it into third (as per the manual) and it will chug along all day. I could never get it to idle along in 4th gear even on flat ground around Broken Hill. I used to sit on about 92 km/hr (100km on the speedo). In 110 km speed zones, I'd sit on 100km/hr which is 110 km on the dial. It was also still fine at 110 km/hr if I had to get somewhere but hey a caravan implies a holiday!