Rear Leaf Springs 7.5t add leaf or convert to air for towing?

Hi all, I have a Iveco 1994 75E15 thats been conveted into motorhome. Its 30' foot long and as such its got quite a long overhang with a garage. I had a tow bar fitted to pull a 4wheel car trailer and am now concerned when we go away with a fully loaded garage and a car on the trailer its puts too much strain on the springs. I'm thinking of upgrading the rear leaf springs (adding a couple more leafs?) or is possible to convert to air suspension and if so how much would it cost?
Thanks
Lee