Return of the Dragon Lords and Torrential Tribute
Should Return of the Dragon Lords had been in the graveyard by the time my opponent activated Torrential Tribute?


Yes your ROTDL was already in the GY before your opponent had chance to activate TT.

This is true because to activate TT the summon has to resolve, and that only happens when ROTDL is already in the GY as the final link in the stack chain. 

Putting it in sequence:


You activate ROTDL, target Red-eyes, opponent passes chance to chain and so do you. Thus, the stack starts to resolve.
ROTDL brings Red-eyes back, then you put it in the GY. The chain resolution ends and the summon is successful.
Given there was a summon, your opponent is able to activate Torrential Tribute. As you can see, ROTDL is in the GY by now...