Even though reaper's crossfade editor is lamentably not accessible at the moment, some reaper and sws actions can help you get the job done easily and in most cases very successfully. You will have to assign them though. Also be aware that some of these actions will perform more than just crossfading, as they intend to help you fill gaps using whatever means necessary (whether it'd be stretching, moving, crossfading) so always check and audition until you have satisfactory results.
=== the item properties dialog ===
Normally you would select the two or more problematic items first, then use any of the following:
*Item: Crossfade any overlapping items
*SWS/AW: Fill gaps between selected items (quick, crossfade using default fade length)
*SWS/AW: Fill gaps between selected items (advanced) Opens a dialog with settings for you to tweak and experiment.
*SWS/AW: Fill gaps between selected items (advanced, use last settings)