There are a couple of different ways to go. I suggest that YOU need to decide how to proceed before you choose someone to do the work.
Probably the easiest will be to buy a set of parabolic springs from All Terrain Warrior in Australia and then get them fitted locally.
Another alternative (and potentially better in my view) is to shift the suspension mounting positions on the chassis and then design and fit longer springs.
But how would I know? I am no 'expert'.
Cheers,
Peter
OKA196 motorhome