I agree with applefreak.
When I did Rome - Florence - Venice - Paris, I booked all my train tickets individually. The Italian website might look confusing at first but its not difficult. check out the website seat61 for some tips on how to book and how to get cheapest pricing.
As for overnight train, it will be more expensive to get a 2 bed cabin compared to a budget flight on the same route. I took the overnight train once (albeit in Germany and paid something like 180 euros per pax for 2 people). That was promotional price for an overnight route in a private cabin. It was not that bad, i.e. we slept though the night. I chose to take the train then only for the experience.
Fast forward to my Italian trip, I took a flight from Venice to Paris this time. If you book early, price is in the range of 100-150 euros each for budget carriers.