Yes you should make it absolutely 100% unambiguous because a lot of sites use this ambiguity intentionally for a kind of bait-and-switch design, and I am extremely wary of it. I think a lot of other people would also be very wary. It's a well known trick to make the price look lower by quoting the one-way deal even though most people are looking for roundtrip.

So.. I'm glad to hear you do mean RT in both cases.

If it's a matter of wanting to avoid clunky long layout for the copy, you could consider using RT instead of roundtrip. With a tooltip on hover, or some other accomodation, for people who need to know what that means.