It is usually advisable that if one of the four coach batteries goes bad, it is best to replace all four at once. I have found that Sams Club or Cosco carry a suitable line of 6 volt deep cycle batteries. I'm sure there are others and the folks on here can be of help there also. On the Tradewinds chart that I have, fuses F17 and F20 are 10 amp and the others on that row are 15 amp. That being said I'm sure those have been replaced over the years on a coach that old and I doubt that 15 amp fuses in those spots would cause a problem.
You mentioned the CO2 sensor beeping. That usually occurs when the house batteries are low. It sounds that many of your symptoms might be due to low batteries. I would suggest a check of each one and if necessary, check your charging system. (inverter/Charger).
Posting the solution, when you find it , will be of great help to others who may have encountered similar problem. If you would like a copy of this fuse chart I have, PM me a cell phone number or address where I can send you a copy.
Happy Travels. Arnold