said just put the plastic cover/plug back in, if difficult spray WD40 or another lubricant on it to ease re-installation. As a retired truck driver I've seen countless numbers of these off and/or missing with no adverse effect on the brakes. Also, as Kuzzy
said, that port is used to release the brakes when there is a brake lockup due to lost air pressure, leaking brake pod, so that the vehicle can be move in an emergency. When you get the opportunity, start your MH, build the air pressure, have someone apply the brake, release the parking brake and you can get under and feel if any air is escaping at the plug port. Trust me you will be able to feel it and if you are not sure shut the engine off with the brakes still applied and parking brake off, this will allow you to hear a leak. If it doesn't leak (I will almost bet it will not) you will be OK.
Good Luck and keep up the eye balling to find things that might be wrong as that effort will bear fruit in reduced repair bills.
Happy New Year
Tom and Barb
2000 Bounder 39Z