FG Falcon rear suspension

I tow an 1800kg (ATM) single-axle caravan with a 2011 FG Falcon. The correct level ride bars are used which are set to get the car pretty level. I'm happy with the car as a tow vehicle, economy and the six-speed auto, but I feel the rear suspension is a little soft, especially over sharply undulating roads. A well-known suspension franchise recommend fitting HD coils and air bags. Does anyone have experience with these or other improvement options?
Also, the six-speed auto has the standard factory fitted cooler. The box is supposed to be "sealed for life". When towing I use it basically as a manual in sports mode and rarely get into sixth. Any traps I should be aware of?