Dave-
I'm not sure what you mean by "outside panel" when you refer to the black water tank bay, so I'll assume you mean the bay access door. When we initially bought our coach, I was a bit befuddled about that access, because it wasn't a typical bay door. On ours, there are two hook latches underneath the access door's bottom lip. Releasing their catches allows the door to swing up. Behind it are the gray and blackwater tanks, several Alladin modules, the solar controller, a hydronic bay heater, temperature sensors, and a myriad of pipes and wires. Our water pump and fittings sit in a white box that distends into the bay from the adjoining bay, so the pump gets some exposure from the bay heat, to prevent freezing. The pump itself, however is not accessible from this rear bay.
If your coach is like ours, there is a small carpeted panel that covers that pump configuration on the adjoining bay's wall. Removing the panel (it easily pops out or in) provides access to the pump, filter, T-connections, and winterizing kit valve. Our rig came from the factory with TV outlets already installed on that same wall. Mounting a TV arm on that particular wall, though, is complicated by the water pump access panel. That's not to say it can't be done. I'm not certain you need a backer plate to mount the arm, as long as the TV is not too big.
By the way, those two hook latches may need adjusting to get them not too tight or too loose. If unfamiliar with them, you may have to play around with the latches to see how they work. Then loosen the small nut just behind the latch's end hook, and turn the screw shaft in or out until you get it so the door latches securely, then snug down the nut. Check this for both latches. If the latches are too loose they can drop down on a bumpy road and let the door open inadvertantly. If they are too tight they can distort the door so it doesn't align properly with adjoining body panels and bay doors.
Hope this is of some help.
-Joel
