IMHO, the Royal sounds best with an OS Recto slant front 412 loaded with V30. This is my favorite combination with the RA100. I have the RA100 combo too that has a partial open back format. It is a bit more closed than the typical combo or open back cabinet. The best combination of speakers in the RA100 combo that did not sound like it was loosing volume or lack of tone was the Celestion Crème Alnico 90W paired up with the G12H75 Creamback. I had tried two creambacks, two Crème 90W, the original pair of V30, blend with the V30. for some reason the 212 combination did not impress me except for the Creame Alnico + Creamback. It is different but sounds great. Still I much prefer the OS Recto 412 with the RA100. It even sounded better with two 412 using similar speakers (I have one loaded with the Creamback 75). I did try the MC90 in a 112 extension cab combined with the Horizontal 212 with v30. Souned great with the Mark V so I had to try this with the other amps. It was OK with the RA100, OK being that I am more in favor of the V30 412 cab. Even the Horizontal 212 by itself was a bit too dark for the RA. The OS Recto 412 slant front cab is not as dark in tone as the 212 cabs (vertical or the horizontal). Not sure how the Roadking 212 would compare relative to the horizontal recto 212.
The only issue I see with the Roadking cabinet, it is 4 ohms mono, or two 8 ohm stereo. That should work for you using the 4 ohm setting on the amp. I have no experience with the Roadking cabs. Even if I did, it is subjective to personal opinion. The RA is not as forgiving as other amps are when pairing it to speaker cabinets. I can understand your issue. Having to carry two cabs plus amp and guitar definitely gets cumbersome. Roadking 212 almost sounds like it would work for you, so it will give you options. IF the V30 + MC90 does not work out, you can swap a speaker from each of the two cabinets you have and connect them in parallel (for 8 ohm load as they are most likely 16ohm speakers), or to stick with the wiring of the Roadking, get just one neo creamback in 8 ohms to replace the MC90. You will still be running a 4 ohm load with that, should be similar to the two cabinets you like paired together..... just an assumption.