I doubt if the problem was too much weight, but without some axle ratings and your actual loaded weight, how could we know? Or for that matter, how would the shop know to say that? It would have to be an extreme overload to actually burst the bags.
How old is the coach? In a motorhome, air bags usually last 18-20+ years. Had the bags cracked from the rubber aging, or did they actually burst apart? Having both go at once seems odd as well - I have to wonder if the real problem was a failed air line for the rear suspension and the new bags maybe not even necessary.
Replacing air bags should not be expensive either, but if you took it to an RV shop you probably paid super-steep labor rates and far more hours than the job really takes. Most any truck shop could have done it for you more reasonably.