Thom, being a recent design, leaving room for the grey tank may be one reason Parkliner
kept the raised dinette floor feature. But if that were the only reason, why didn't they raise the whole floor, as sticky manufacturers do? Or, for that matter, as Bigfoot, Oliver
, and Casita
(17' models) do? It's simpler and stronger to build a frame without variations in floor height. And the only answer I can think of is to maintain headroom while lowering the towing profile. And that is the same reason Boler
did it that way in the 60's, when "grey water systems" consisted of buckets outside the trailer.
I'm probably splitting hairs here... but I don't think the step-up exists because of
the water tanks (which the OP's question suggested). It exists to minimize the height of the trailer. However, given a step-up or drop-floor design, the raised floor at the back follows as the only feasible place to mount an external grey tank.