Hi guy;
I have a 2007 4-Runner, 6 cyl. same as you, and I use it to pull a 23' Hi-Lo Classic with a Gross wt. of 6000 lbs. I usually pull it at 5000 +/- 300.
I do have an aftermarket tranny cooler on it, which I do to all my cars regardless of towing or not. I have replaced the weight carrying hitch with a Husky wt. dist. hitch, and have installed two ' 3/8"X 7" eye bolts' using the OEM hitch screw anchor nuts. The bolts go down to a 3/8"X 8" Grade 5 steel bar, that goes under the Husky hitch. reason?
The hitch is rated for 6000/600 lbs. and my tongue weight is around 750 lbs. So the extra support is helpful here. If you send me an e-mail address, I can send some photos to you of them.
I have found a lot of play between the hitch shank and the receiver, so I use two .060 shims, one each, top and side, drilled hole for pin, giving me .004" clearance. I grease the shank for installation, to preclude wear, and it works just super. No noise, no clanking, no wear. Been doing this for six years now.
I also replaced my 265-70 tires, (when it was time) with 255-70, (slightly smaller dia.) to help the tranny and drivetrain to handle a full load. The speedo is out about 2%, that's all; and in 4th. gear, 2500 RPM gives me 95 KPH. true. (gps reading) On our 400 series highways, I can go 100 KPH. at 2650 RPM.
I have been towing this Hi-Lo for about six years now, and the 4-Runner handles it just fine, including going up Vermont's mountains at 95 D. F temps.
The Hi-Lo is a very nice 'home' away from home, and the only other RV I would desire is an Airstream; but too expensive.