On those coaches with hinged compartment doors instead of Smartbeds, those "solid panels" are hinged like the other doors. They are just bolted shut with sheet metal screws from undernieth. You can remove the bolts and get to the holding tanks. We put latches on ours to make access easier. (As well as the sheet metal screws kept falling out.)
I assume those panels on Smartbed coaches are also hinged.
'04 Alpine Coach 36' MDDS