Hi Kavman.
I have a Hymer 644 on a Ducato - 1998.
3 years ago I had the same problem as you. Sagging leaf springs, tiring to drive, bad cornering etc. I checked out the cost of replacing the springs but it would have come close to 1000 euro all in.
After a lot of research I bought Dunlop air suspension from Marcle leisure in the U.K.
http://www.marcleleisure.co.uk/store/-c-58.html. Cost about 350 euro
My local mechanic fitted them for 100 euro. The old leaf springs stay in place with the 'stopper' taken out (I could probably have fitted them myself by my local mechanic always gives the van a good inspection when I drop in). I didn't bother getting a compressor kit with them as I just use my 12v pump that I carry in the van anyway.
The difference in the ride quality, cornering and ease of driving is huge. You can set the pressure in the airbags at different psi for different roads, loads etc. On motorways I put about 60 psi in. On back roads a little less.
When I level up, I usually have to go down to about 30 as the back of the van will be too high if I don't.
My advice is to get airbags fitted. They're cheaper, more adaptable and get rid of all the squeaky noises.
Have a look at this link which shows a set being fitted - exactly the same as mine.
http://www.fiatforum.com/misce…rhome.html
Hope this helps
Denis