There is no reason to carry a camping toilet with you when you are in Italy.
If you are travelling in an InterCity train which will not stop at Roma S. Pietro (Vatican) you may find these toilets:
If you are travelling with a RegionalTrain which also call Roma S. Pietro - it looks this way:
If you are in Roma Termini-Station you just follow this sign:
So have 0,70 EUR with you in 50 Cent, 20 Cent, 10 Cent and 5 Cent coins with you. A change machine is just in front of the entrance area.
Good luck in Rome.