Here is the section from the Rick Steves tour page on flights:
Flights
Our tours don't include airfare, so your flight arrangements and transfers are completely up to you.
Most tour members arrange to fly into Ljubljana (LJU) and out of Dubrovnik (DBV) — choose "multicity" on flight-booking websites. Booking round-trip to one city will require a connecting flight, train trip, or additional overnight stay in Croatia.
We suggest arriving a day or two before your tour begins to allow ample time to get over jet lag and be well-rested for the first few days of your tour. For more information about transportation options to get to and from your tour, see your Rick Steves Croatia & Slovenia guidebook.
I just ran a quick check via Google Flights routing you from LAX to LJU with a return from DBV. There are plenty of flights including several one stop flights on AF, BA, Turkish Airlines etc. Your return flights seem to want to route you back through Zagreb. Definitely add additional days to your trip for travel.