[Richard]

Well, this is turning into an exercise on what to do.
I was a fan of the Firestone Bags, but obsolete is obsolete.

I did a great deal of digging in this Goodyear catalog  https://infinityairsprings.com/wp-conten...ate_v1.pdf

First thing, there is a phone number on the catalog for calling Infinityairsprings. That might be worth a shot instead of dealing with middlemen. 

Second thing I took the critical dimensions of the stud spacing, bag height extended and compressed, and tried to find alternatives that would not require much modification to work. I found some that would require new top holes closer together. All the alternatives had 1/4 npt openings.

Third thing, I moved on the look at 1 inch larger diameter bags. There appears to be plenty of room to fit them. There are two other reasons to consider larger air bags. One, is the air bag sizing from front to rear on the Ridewell rear/ DANA front coaches is out of balance. For example it takes about 80 psi to raise the front of my coach, and 110 to raise the rear. This is all due to the weight to diameter of the bag calculation. There have been instances where folks had air compressor governors fail, or air compressors start to weaken, and they could not develop 110 psi to raise the rear of the coach. The second reason is that all the alternatives I can find have a 1/4 or 3/8 npt air inlet. That means that the ping tank is going to be bypassed. I cannot quantify this but that will increase the spring rate when the ping tank is bypassed. Increasing the diameter of the bag will restore some of the spring rate. i.e. it will be softer.

So, to the best of my ability I was able to identify 12 inch diameter bags that will fit according to the dimensions in the catalog. The drive axle would take a 1R12-350 and the tag will take a 1R12-558. A couple of things to note. The drive bag will now have the air inlet as part of one of the studs. Also DO NOT put 12 inch diameter bags on the drive and leave the 11 inch diameter bags on the tag. This will put more weight on the drive and less on the tag then currently exists. The tag is already underweighted. If you don't keep the diameter of the bags the same I am afraid the tag tires will slide in a hard braking situation.

If I were a good boy I would have called Infinityairspring to see if those were more readily available, but we leave in the morning, and I am a bit busy. Also, the catalog lists cross reference manufacturers of those bags which may help in sourcing.