Why would I have i lost one leg on 240 after maintenance guy changed 100 amp disconnect at the meter
To me it sounds like one of your hot leads to the mobile home was damaged. Turn every breaker off but leave the main on and you will probably see one leg is dead.
My guess is with the smaller breakers on there was enough pass through to provide a voltage, remove the opportunity for this to happen and the bad leg shows up.
How can this happen? I’m my experience it is usually the water heater turning on or trying to so there is voltage on both legs but no potential difference. Open or turn all breakers off and that will usually provide the dead leg.
Someone working with a live circuit like that should have the training and gear to do it safety it is obvious they don’t and should not have done this.