I've been towing my 2500lbs popup camper with my '06 Highlander AWD V6 for 5 years now. I've been a member of this group for some time, but we've only bought our first FG trailer last summer. Our FG trailer is a Trillium
5500, I expect the loaded weight
will be 3300-3500 lbs. I've only towed the Trillium
about 100 miles the day I bought it. Everything went just fine. The trailer is currently being restored/redecorated, probably a 2 years project
I've towed the popup 2-3000 miles every summer with the Highlander. My HL has the towing package as standard equipment, just like all canadian-market AWD V6 Highlanders (and Siennas too I've heard). You can confirm the presence of the transmission cooler, it is visible in the front driver-side corner "air intake" below the bumper.
Always turn off Overdrive when you tow. I use Mobil 1 ATF, synthetics in the engine, transfer case and rear differential.
The rear suspension of the HL is rather soft, and my popup has a tongue weight
around 300 lbs, depending on propane
quantity. I use Reese's 350 Mini weight distributing hitch (now discontinued) which also acts as a friction anti-sway. Good system, trailer is super stable behind the HL, and the HL rides level. I would not tow anything but a light
utility trailer without the WD hitch, as is improves the ride dramatically, it's really night and day.
If I was you, I'd forget about helper springs and I'd invest in a similar WDH system (combined weight distributing and anti-sway) like Reese's SteadiFlex, Husky CenterLine, or Equal-i-zer, with a rating appropriate to your tongue weight.
Make sure the receiver hitch you have is rated for weight distributing. First, it has to be a Class III (5000 lbs), but depending on years, some Curt hitches were not rated for WD, and on some years it was hitches made by Cequent (Reese, Hidden-Hitch, Draw-Tite) that were not rated for WD. My HL came with a Cequent hitch not rated for WD, I had to replace it with a WD-rated Curt. See my posts in this thread on ToyotaNation:
Advice on trailer hitch for 2002, best looking hitch? - Toyota Nation Forum : Toyota Car and Truck Forums
Sometime around 2004-2005 Toyota introduced the 7 seat version HL. Because of this, changes were made to the rear frame of the vehicle, and I think this caused some hitches incompatibilities between model years. If yours is like mine, Cequent Class III hitches are not WD rated, but Curt and Valley hitches are.
The FG trailer we bought came with a very old WD hitch. It's a standard model with chains and blade-type friction sway-bay. There is no brand name on it, I've never seen anything like it, and it looks waaay too stiff for my trailer, so I don't like it and I intend to eventually ditch it and get a newer, more efficient system, like an Equal-i-zer. But the Highlander was towing the Trillium
just fine with it. Aerodynamic drag is much more noticeable than with the popup, but I expected this.
Back to the HL: if yours is like mine, there is a big connector hidden somewhere under the plastic panels and trims at the rear of the vehicle (on 2006 models it's on the driver side). Toyota's OEM converter and harness kit plugs in there. But this kit costs a fortune, and is only a 4-pin harness. I bought an aftermarket converter and did my own wiring. All the wires you need are at this connector, you can easily figure out which is which with a simple test light
Also, Toyota's converter is a powered unit, this means the trailer lights
are not powered from the HL's light circuits, they are powered straight from the battery
, with the light circuits of the HL only providing "signals" to the converter. This is much better than having like 20-something marker lights
drawing power to a car's light system that wasn't design to handle that load. You can also get aftermarket "powered" converted, they are a bit more expensive than the regular ones, but they are worth it, and this is what I'm using. No need to run a large wire from the battery to the converter to power it, as you can use the same wire that powers the OEM converter, in that same connector. This wire is powered when you insert a fuse in the "Towing" socket in the engine compartment fuse box.
My brake controller is a Tekonsha Primus IQ, very good proportional unit, and quite affordable. Get anything from Tekonsha and you'll be all right, except the old Voyager model, which uses old technology.
You will need towing mirrors, I use 2 Tow-N-See convex, the work very well.