overnight train from Rome to Venice

I would not encourage a sleeper train. A Eurostar train from Rome to Venice is 4.5 hours. There is a train departing at 22:50 that arrives in Venice at 5:24 in the morning without changing trains. Keep in mind: a) you probably will not sleep well, if at all, on the train and b) what the heck are you going to do in Venice at 5:30 in the morning? If you're staying a full service hotel you can probably at least drop your bags off but I doubt you'll get access to your room. In my experience night trains land up costing you a day because you need to sleep when you arrive. You may save a night in a hotel but it will land up costing you the majority of a day of touring.

The train Sam mentions is ICN 774. It leaves from Roma Tiburtina at 22:50 and arrives at Venezia Santa Lucia at 05:24. I like night trains and even sleep fairly well in a reclining seat (although I'd certainly recommend that you get at least a bunk in a couchette). So what to do when you arrive? Find a place for coffee and a light breakfast and watch the sun come up in Venice! Find your hotel and ask to store your bags until your room is ready. Or store them at the station for a few hours and go for a walk.