Let's say I'm a non-EU national who has a Schengen visa type C for tourism. I want to visit multiple Schengen member states. My first port of entry is Italy (for example) and I booked for two way round trip ticket for Italy (which means my last port of departure will be Italy as well).
So while traveling to Germany (for example) from Italy will they require me to have a return / onward ticket from Germany?
Technically speaking you don't even need a return ticket when traveling to the Schengen Area. The relevant regulation (Schengen Border Code, Article 6) only lists the following related requirement:
(c) they justify the purpose and conditions of the intended stay, and they have sufficient means of subsistence, both for the duration of the intended stay and for the return to their country of origin or transit to a third country into which they are certain to be admitted, or are in a position to acquire such means lawfully;
As for traveling between the Schengen members states - no, it's not necessary to have an onward ticket of any kind. You do need to have a valid visa and you can't exceed the 90/180 limit, but you won't pass border controls and you won't be asked for any sort of supporting documentation. Such checks are only done at the external border.