I'll make some additional assumptions based upon info given in O/P.
'O5' unit would; with options mentioned, almost certainly be a 50 ampere rig.
Also mentioned is a genset so will assume his rig is generator prepped rather than one necessitating unplugging the umbilical and using an adaptor to plug into the genny each time you use it.
Based upon those "assumptions" all 50 amp, genny prepped rigs have two legs of service within the service box breaker panel AND a 50amp or higher rated "transfer switching device" somewhere in the service bay of the rig.
A frequent occurrence with all RV units is poor assembly protocols leaving contact screws loose or road vibration causing the same loose connections to happen.
You need to isolate your rig from all power sources then do a contact audit of ALL connections in your breaker box first (easiest to eliminate quickly) and that transfer switch. If the transfer switch is the commonly used "Iota" brand, just follow your major 50amp service wire back from your breaker panel where it will come from the transfer switch/service box OR follow your incoming supply line through the belly of the beast until it arrives at that transfer switch/box before continuing on to your breaker panel.
Removing the top of that transfer box will reveal a double row of contact screws numbering from 12 to 24 in all and any one of them being loose on the outlet side of the buss bars could cause your issue as it would reduce your rig back to 50A/120V from a 50A/240V supply thereby rendering one half of your breaker panel inoperative. I target the outgoing bussbar in the transfer switch because it would behave as described regardless if incoming was from shore power or genset.
I'm going with this assumption based upon more than one of your larger draw 120 items being dead as it is unlikely they would put a couple of those together on one breaker and it would be unusual for more than one breaker to become loose at exactly the same time.
You need to address this
ASAP as loose/arcing connections at the 50amp incoming to outgoing in the transfer box or at the breaker panel are a major fire hazard indeed.
Wherever you find those loose screws you will almost certainly find heat/melt/blackened degraded plastic insulation for an inch or two on the associated wiring...your call if it's bad enough to indicate brittle copper in the wire itself.
Should you find your problem in and be uncertain if the short sections of copper wiring within the Transfer switch need replacement consult any electrician in your RV fraternity as to how to determine copper degradation but usually a simple test is to take that portion of the wire exposed for the connection and bend it a couple of times to and fro...if it breaks off easily it needs to be replaced along with it's brothers.
Re-cap: ISOLATE from any 120/240 supply either shore power OR genny. - - Follow your major incoming 50 amp wire and tighten every connection you find along it's path to your breaker panel.- - Don't delay your troubleshooting/repair as while it's not functioning it could still be arcing.
****EDIT PS**** Neglected to mention that if rig is 50AMP with a large round Female incoming socket for the umbilical, the rear of that female receptacle and one loose screw could also be why one leg has dropped out of service.
Good luck.
Today is just the tomorrow you worried about yesterday!
