I've only ever fitted one on a outback van (can't remember the model) customer supplied one of those chain type spare wheel carriers that you wind down, all we did was weld 100x50 RHS across rear chassis up against floor for mounting plus a few plates to locate the wheel and informed owner that we were not liable for any damage it could do, he was fine with that, personally i think it will be fine as rear water tanks are mounted like that across the chassis and they hold more weight than a spare and i've never seen damage from them, i think the rear bar mounting is like putting something on the end of a seesaw and it tends to bounce which causes the metal fatigue and on saying that i've seen heaps of vans with big storage boxes and jerry holders mounted on bar with no problems, could be effected by the driver and road conditions to some extent