Your going way back in time with a 20MT.
Always easy first check the battery voltage drop, those are not like the newer lifts that cut off with a bad battery.

If I remember right you should have 4 batteries 2 for the lift and 2 for the travel.

EDIT if the travel 2 batteries are good then it would travel fine even when your trying to lift a heavy load if the 2 lift batteries have a big volt drop under load then it would make it struggle lifting a load.