When I considered getting an enclosed 6x12 v-nose trailer to haul my bikes to track days and what not, I drove and sampled a buddy's trailer first. I had just installed the 1 inch front leveling kit and hooked up the trailer semi- empty. Weight was around 1500 lbs.
To be blunt, with the front level kit, it sagged a lot in the rear and you could feel the 4runner struggling and I did a 30 minute or so highway test ga400 North with some rolling long hills around 65mph. It didnt feel good and being very gentle it was averaging 9.5 mpg.

I installed some air lift air bags, went back to test the same setup again and the sag ALMOST went away completely, it felt much better and didn't struggle nearly as bad and I averaged 14 mpg replicating the same test from last time.

I went and picked up my own 6x12 and have used it to haul 2 bikes and all kinds of stuff in the trailer numerous times. Probably around 2500 lbs.
Last 250 mile trip I made with this configuration (airbags) to Alabama, I saw 13.5-14 mpg.

If you're gonna tow with it, especially above 1000 lbs, I HIGHLY recommend getting air bags to help. They make an immesne difference. The V6 really will start to struggle around 3000lbs+, though. Above that, I'd want a V8 and a longer wheelbase.

I've been towing with it from 140k miles to now - around 185k and it hasn't seen any problems from towing. They oem radiator finally sprung a big leak at 180k, but it's a 4runner and I don't think hauling 2500 ish lbs is going hurt it more than it will be naturally hurting itself.

First time trying to tow without airbags vs after






Air Bags top
Without on bottom
(not a huge diff in photos but huge diff in real life)