T-Mobile's international roaming data works fine, but note that data speeds are supposed to be capped at slow "2G" speeds. (Some people report faster rates than that.) That should be fast enough for using Google Maps. It wouldn't hurt to download the maps for Croatia, Bosnia, and Slovenia ahead of time "offline" as suggested above, if your phone has enough storage space. Then, if there is some issue with mobile data, you can keep using the phone as a GPS, even with no data connection.
Be sure to bring a car charger to plug into the "cigarette lighter" power port, as using the phone for GPS/maps can eat the battery quickly.
Also, unrelated to the phone: be sure to remember to buy a vignette driving sticker at the Slovenian border to be able to drive on Slovenian highways or you risk a steep fine if caught without one!
And, you are technically supposed to have an International Driving Permit (IDP) to drive in Slovenia (maybe Croatia, maybe not) in addition to your US driver's license. The rental car people may not ask for it, but the cops might if you ever stopped. $20 USD at AAA before you leave - a wise idea to have it.