Parking is at a premium at both the subway and commuter rail lots. In typical Mass. planning, they didn't anticipate how many people would use mass transit. The lot in Braintree and at Riverside are full by 8:00 am. The person you talked to at the T has never actually had to commute via the T.
If I were doing this based on the fact that I've lived here for 50 years, I'd grab a spot at Normandy Farms or Minute Man, drop the trailer and drive to the commuter rail to park just the truck. Alternately, you could grab a cab from the CG to the commuter rail station. I seriously doubt that you'd find a parking spot for the truck and trailer at any T lot.