I want to get an M13 to use with my Roadster, which is almost the same amp as the Road King. I can't think of any reason it wouldn't work.
Ideally, I'd hook it up to my amp using the "4 cable method". That is: Guitar>M13 / M13 Effects Send>Amp / Amp Effects Send>M13 Return / M13 Output>Amp's Return. This way, I would have the option of placing any of the M13's effects either before or after the Mesa's preamp. The M13 lets you assign the position of it's Effects Loop per "scene". So you could, for example, place an M13 wah before the loop, go to the amp for the real tube amp gain, and return to the M13 for delay or modulation effects.
I'm hoping I can use the M13's Scene Mode with a Mini Amp Gizmo to automatically switch to the appropriate amp channel with M13 patch changes.