We overnighted Paris to Florence about 5 years ago. The trains were Italian we were in a shared compartment with 6 others. . The departing train was very late so we spent ages waiting at an uncomfortable Paris Bercy but the trains now depart from gare de Lyon now. The shared compartment was a tad run down and you needed to work out how to errect the bunks and reach a group consensus as to when to do so basically seat makes bottom back rest middle luggage rack top.
Fairly comfortable to sleep on . take food and lots of water to drink, there was no evidence of a buffet car a guard gave out a free small bottle of water and some odd bread crisps about an hour before we were due to get off in Florence . On the return journey the guard advised us to lock the doors to avoid problems with undesirables .
My wife has done the same trip on the expensive ticket which was much more salubrious and avoided the enforcement proximity to a French students feet.
The trip was an experience made better by using it as an excuse for a long weekend in Paris . Longer but more fun than air travel.