Our coach is a little older and may be different from yours. Our diagnostic plug is under the dash just to the left of the steering column. There's a carpet covered vertical panel (footrest?) that must be removed to get to the plug. That area also houses the prewire for the CB. Our panel is held in place by screws through a bracket into the floor in 2 different places.
Hope this helps.
Denny, Connie & Shadow (former barn cat made good)
2000 Itasca Horizon, Cat 3126B, Freightliner Chassis