You need a cross bar to keep the brackets and the arms of the tow bar separated under the strain of the tow. Blue Ox base plates have the cross bar built in and mounted behind the grill, while Roadmaster puts them on the outside (the cross bar comes with the tow bar).
I'm not clear on how the cross bar that comes with each Roadmaster tow bar would bolt to an MX bracket. The cross bar mounts I am familiar with would not fit the slots on the MX. The Roadmaster tow bar instructions say the tow bar connects direct to the MX, which kind of implies that the cross bar is not used. All the pictures in the towbar install instructions show a cross bar, but only with a non-MX bracket. I'd suggest a call to Roadmaster to get the word right from the source. Maybe they only make the MX brackets for a vehicle that has strong cross bracing already present in the front end and therefore doesn't need a cross bar.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL