The Marittima is a 10-15 minute walk from the Piazzale Roma. There may be a free shuttle between these two points (I'm not sure). Once at the Piazzale Roma, I know that you can take the vaporetto one stop to the Ferrovia stop which is the train station.
Alternatively, the new 4th bridge over the Grand Canal has opened. It's supposed to permit you to walk from the Piazzale Roma to the main train station but I haven't yet seen reports on this new bridge. I've seen night pictures on Youtube and all I see are stairs up and stairs down. No ramps for transporting luggage. Has anybody made this walk? If yes, how was it?