Your tag is probably sitting on a More-Ride rubber suspension, which may be badly compressed if it looks like no suspension. You didn't give a year coach so not positive. Our 90 Airex we had was equipped with a tag axle mounted on this system.
To switch to an actual airbag suspension will get kind of pricey very quickly. There is one talked about on here from time to time that begins with a K. Can't remember the name right now.
Anything else will just make it harder as the add-on airbags are for added load capacity not smoothing the ride out.