Sagging rear - Replacing spring springs with stiffer ones?
 

My 09 35d w/ Adaptive Drive sags just a little when I tow my boat. I recently switched to non run flat tires and things are a softer in general even with the 19" tires in the +40 psi range. I am wondering if I can switch to a stiffer rear spring to stiffen things up a bit?

Ideally I would like to add the factory self leveling rear suspension however nobody has successfully done that yet. I would even settle for a manual system where I just added air when towing (keeping the non self levering springs). At some point I need to crawl under there and see it there is any wiring at all for the factory system. My E70 is fully loaded and only missing the 3rd row so there is a very slight chance enough of the wiring is there.

Any advice is appreciated. Thanks

How much tongue weight do you have in that picture???

It is probably 400-500 lbs in that pic. The boat is empty with no gas. The trailer is OEM Yamaha and comes with the boat from the factory. It is not some universal trailer that was set up by a local boat seller. The boat weighs 3000 lbs empty and the trailer is 1200 lbs. It looks heavier than it is. Yamaha's are much lighter than normal boats as they have 2 1050 cc engines that are pretty light. Most 23' IOs are over 4000 lbs.

Aftermarket airbag system?