Landrover Discovery rear 'air-bag springs'

I'm probably calling it the wrong thing, but my Disco (yr 2000), in common with many Discos have the 'air-bag' type 'spings' on the rear of the car.  They are 'self adjusting' - or rather should be.

I have a problem with mine at present, which means I could have a leak in the bag, an air leak in the system, sticky valve, or sensor problem.  I took it to my mechanic this morning who will have the car to check it all out.  If it looks like being an expensive job, he suggested one alternative might be to replace the 'air-bags'  with traditional springs.

Has anyone had this done, and are there any potential problems with towing.  Mechanic reckons not - he said he had an old Range Rover with springs and had no problems with towing, and that a noseweight of 80-90kg shouldn't make an appreciable difference.  He said, basically, stick three people in the back and fill up the boot and this would make more effect on the 'downward' pressure on the car than the 80-90 kg noseweight when towing a caravan.

So basically, has anyone replaced the airbags on their Disc with traditional springs and does it make any appreciable difference to towing a caravan?

David