OK Marcelo, Hang on Tight.....
What you have is a "Rear Bath Birch". Some find this a very desirable version. Be sure that any modifications you make actually make it better. Please add that note to the sigfile as it matters a lot. Birches are very different and some things matter a lot.
I do hope someone advised you that a reliable and ready travel GMC will cost you about 30K$us. What does not get said enough is that this is all at once, or as a kit and sweat gets billed in at full shop rate. Places that will work on her are probably getting 100+$us/hour out there. With that kind of cost, you should consider buying any tools you might need and just expect that they will amortize in short order. Besides, if you do the work, you know it got done right, or at least you know how to do it again.
If you do not have a maintenance log, start one. If you do this right, before very long at all, this will all be a blur. Vehicles of this period all required regular maintenance to stay reliable. She can be reliable, you will hear of many that do extended excursions with no issues. If you do this log in digital, be sure that there is an accessible cloud back-up. I also have another list that I like a lot. Chaumière Needs is a six part list. First is "Needed Now", things that have priority and need attention. Second is "Winter Jobs" are the things that will happen when she is not busy going places. Third is "Waiting" things that have low priority but I don't want to forget. The Fourth "Deferred" just makes them lower priority (May it need materials I have to buy).
is "Abandon" those thing I thought we be good but now have doubts are of any value, but I don't want to forget about. The last is a fun one. It is "Done". This is good because went something is done, add a date and move it here and it is very rewarding to look at the growing done list. This you can keep in digital as it won't matter if it gets lost.
The Airsprings are not nearly stock. From the bit of the picture, they look like Ferrara bags. Many people are happy with them. As Christo said, air pressure means nothing. This is particularly true of a light (yours is) 23 coach. If you are doing longer runs, leave the valves open and left the system work. You are carrying nearly 300# of potable water that moves forward with flushes, and about the same weight in fuel that will be gone in about 400 miles. All of this upsets the coach fore-aft trim and can lead to handling issues that you will not be able to identify until you have several thousand miles under you.
Being a rear bath, makes the potable tank more difficult to remove- IIRC. So, I am going to suggest that you get under the coach (NEVER get under it when it is not blocked up) and poke at the area you think may be a problem with something sharp and try to ascertain just how bad the flooring is. You may find that it is different than you thought.
About being under the coach. As said above, do not unless it is blocked or something. When the rear suspension fails. the back of the coach can crush a snake. Really, if you ever see this happen, it is scary.
If you do not have all the owner's and service documentation, you should. It can all be downloaded at GMCMI.com. Or, you can join GMCMI and you will get, that on a thumbdrive and the magazine and other good stuff. You should also look up Cascaders and Western States and join up with them. Next year's fall international is planned for Rapid City. Make plans now. When you get there, be very ready. I have been told by others that I got to go there that being at an International for the first time is a lot like trying to get a sip of water from a fire hose. A Western States in much the same, but I have only been to one of those (so far).
There are two forums, and they are both very good. This is the newer one, and has mostly newer owners. The older one (GMCnet) was started as a e-mail group in the early 80's and has a forum as a mirror of that group and so it can be difficult at times. That is were the long time owners write. Some are original owners and many have worn out a coach or two. There is also considerable coverage on Facebook. Do Not Take any advice there on presumption of accuracy. I have seen at least two coaches end up as scrap because the owner took bad advice.
About batteries (before I forget),disconnect the rear one and see if the generator (APU) cranks. If it does not, then that is what that battery does.
About the coach, it is much more than a big new toy. The most important thing is the community it connects you to.... Between here and the other forum, you just collected a couple of thousand new old friends. This community can be off-putting until you realize that most everybody here has a not so secret agenda. That agenda is to see to it that you get to enjoy both the coach and what it can do for you. We all know. It is a community of supportive and helping people the like of which you most likely have never experienced. Let it work for you. There is an amazing resource that is waiting at these forums and the local owners groups. When I discovered this I knew what to think. It is very much like the community of watermen that are my world. For this reason, (and because I now have a name), I like to welcome new owners much as you might be welcomed there. So,
May the Good Lord bless this coach and all those that set forth withing her.
Welcome Marcelo
Matt