... Fix UVs?

RealFlow's built-in objects have UV coordinates applied, but they are not relaxed. A “Cube” object's UVs, for example, are overlapping, and you have to use an appropriate projection method like BodyPaint 3D's “Box” mode to separate the surfaces. When you re-import objects which have been created in Cinema 4D then it is in your own responsibility to check for correct UVs. RealFlow does not provide tools for manipulating UVs.

If the SD file contains objects which have been created with other 3D packages and then used inside RealFlow you either have to ask the creator of these objects to fix their UVs, or you use BodyPaint 3D to correct them. If an object does not contain any UV information at all it is possible to do all UV-related work inside Cinema 4D, and finally create/assign new coordinates.