Poly Air Springs for 130"

I don't know if I'm right, but as I understand it the inner 'helper' springs in the rear of the 130 are there to cope with heavier loads with affecting the ride when unladen.

It's pretty much a moot point if the van back is on permanently, your dad may as well get a set of heavier springs to cope with the heavier permanent load.

If the van back comes on and off, maybe you could have a heavier set of inner springs made up, then you wouldn't lose any of the ride characteristics without the van on.

Cheers
Simon.