After talking to many gearbox people over the last few weeks the general idea is that it's fine to tow in 5th but if it's a heavy load (1.5t and up) then 4th is a better option when going up hill.
Having a chip, exhaust, bigger tyres, and 2.5t of boat then planting the foot in 5th to get up hills is probably not the best idea for gearbox longevity. I may have been guilty of this practice in the past
On a side note not using 5th for the last month or so and still doing a few long trips as well as towing the boat my fuel usage has not changed at all. 9.5-10.5l/100 not towing and closer to 12 when towing.