Yes, the Roadmaster to Blue Ox base plate kit adapter part # RM-032 that you referenced does work with the Roadmaster Crossbar base plate kit part # 1208-1.
You should note that this adapter was designed to work with Blue Ox tow bars so you would need to verify that's what you have. view full answer...
To attach to your round crossbars you will want either the Rhino-Rack Pioneer Platform in a 48"x38" Kit # RR42114BF or the 58"x47" Kit # RR42115BF. The Platform # RR42TB you found is designed to mount with either a set of feet or to the Rhino-Rack heavy duty crossbars using the Hardware # RR43105 view full answer...