bt50

ask Mazda if you want a qualified answer
Mitsubishi gave me a letter saying the 3.2L Triton is rated to tow rated load in 5th gear.

Ford Ranger(same vehicle) owners manual says don't tow in overdrive. We had no problem towing boat and tray top camper to SA and back August 07 with Ford Ranger auto in 4th gear.

Mazda advise sounds OK not labouring the engine sound logical. Our Ranger is a 4x4 auto TD so not sure if that makes a difference, sorry can't be more technical.

Is it Auto or Man?
Generally, the main issue is that overdrive need to to use multiple gears, where as forth gear is straight thru the center of g/box, that is to say, basically no gears are being used, just a shaft straight thru.
Some gearbox's use and idler gear and Third to achieve overdrive, thus loading up thrust bearings and planetary shafts, all of which create heat.
I would guess that Fifth in your BT50 is most likely Overdrive (auto or manual) and I wouldn't worry about making the motor work (peak torque around 2000RPM) any less than that change down,
That big caravan may tow best in 4Th IMHO, come to a hill and you will have plenty of rev's in reserve thus not needing to be constantly changing down (or "hunting"in auto ),Flat road ,no headwind , 5th may be more suitable, this rule of thumb applies across the board, not just to Mazda/Ford.
There are plenty out there who tow in fifth with out worry, there are also some out there who have shelled out big $$$$ for g/box rebuild, each to ther own, I had to rebuild a g/box at 80k and never towed anything, you either get a good one, or not.
Shane