Wires on the top speaker should cross over each other on the bottom speaker. "+" to "+" and "-" to "-". Each side (left and right) should look identical. Also the tabs of the speakers should be facing each other to keep the wires shorter. The difference is in the jack plate with the 4 jacks mounted on it. Did you leave the solid bent wires alone? I saw this link on 412 cab wiring. There is an image of the speaker plug plate in the thread. The black wire connections are hard to see but those solder to the lugs closest to the mouting plate. Also, have you checked impedance with a meter? It will not reveal a phase issue. I would also check the switch function on the Metal jack to verify it works with a plug inserted. Leave it unused for stereo mode.
http://forum.grailtone.com/viewtopic.php?f=4&t=35293
If the speakers sound okay with only one amp, using half of the cab, and the other using the other half by itself, If either amp lacks the bottom end you may have a phase issue with the one side or both. how does the cab sound in mono mode?
If it sounds weaker than you may have the speakers out of phase. Either that, the amps may be out of phase with each other when using them together for stereo. Are you using the FX loop on your Road king ? Not sure if that does the same phase shift like the Mark V does. Last time I ran two amps in the Recto cab it was with a Mark III and Mark IV. If you ran both the combo King and the Marshall before and no issue, could be phasing on the speaker wires.
If this is the first time you had phase issue, not sure if it is the CB speaker, I doubt they are out of phase with the other two. I would check wiring if you miss wired the speakers. Might sound honky if they are out of phase. No harm done but usually does not sound good. I did that once when working in a dark room. OOPS. yes, it can happen to anybody. Also if you did not reuse the original wires, best if the + and - are different colors. For Mono Stereo, I would probably use blue for the - on the one side, and red for the + on the other. That way it is easy to see what is what.