That's how mine (2004) is oriented as well, and it rides/handles fine. However, I have the battery on opposite side and an air spring kit like this one , which has been a fantastic help to keep the rear suspension fairly level side to side as well as front to rear.
2018 Highlander XLE V6 AWD
Had a shop put the tow-hitch on with a proper 7-pin and brake controller.
I put in the airbags myself (AIR LIFT 60732 1000 Series Rear Air Spring Kit). These are for a Sienna but fit inside the coil springs. I keep 5 PSI in them at a minimum and towed with them at 15/20 PSI. You don't have to take the suspension off (just remove the tires). Just have to lift the flap that covers the rear springs and use an angle socket to remove the nut on the bump-stops. You can use pry bars to separate the springs enough to get them out but it's a bit of a bitch so be prepared to cuss. Once they were in, I ran the tubing up to the small flap in the back (not the brake light flaps). You can get fancy and mount an air gauge if you want but I only towed for the summer as we bought a truck in prep of a bigger trailer as we camp a lot with the kiddos.
Yes I installed myself. Etrailers.com has great instructional videos. You can buy them from Amazon for $95 if you can wait a couple weeks. If it’s gotta be quick etrailers.com has them for a little more and if you need it they have quicker more expensive shipping options.
Here they are on Amazon
AIR LIFT 60732 1000 Series Rear Air Spring Kit https://smile.amazon.com/dp/B000CFS028/ref=cm_sw_r_cp_api_glt_fabc_QK580QRVFFKN17BRW6RK
AIR LIFT 60732 1000 Series Rear Air Spring Kit by Amazon.com Learn more: https://www.amazon.com/dp/B000CFS028/ref=cm_sw_em_r_mt_dp_CKBZFbGTV1JBS