Please help. 84 fleetwood southwind brakes

We just bought an 84 fleetwood southwind 23' motorhome. The previous owners told us that it was going to need a new master cylinder. My husband has does this before on other vehicles so we thought it would be no big deal. When he got under there it seemed to be leaking where it meets what we are now realizing to be a hydro booster. The booster has a small chip in it so we are thinking that it needs replaced instead. We couldn't find the parts identification sticker and the parts store said that it could be a JB8 or JD7. We still aren't 100% sure which one is ours but we ordered the JB8. Then we started reading up on replacing the part and it seemed way out of our league. I tried to decode our VIN 1GBJP37W4E3340208 per the title which just made me even more unsure. Does anyone know if I got the right part and if this is even going to be a project I can do in the parking lot? I just bought it and haven't even been able to drive it home yet.

I also read in the vehicle that we have air coil something or other that should be kept at 50 psi. Where do I check and fill that system?

Thank you from a family at its wits end