I like the Fiamma Ultra-Box line of cargo carriers:
The "back" boxes come in 4 sizes. The photo above is the smallest size.
They'd be useful on Class B's like the RT 200 and B+ units without rear doors if you need to keep your hitch free for other uses.
They mount to Fiamma's "Carry-Bike" system but there is also an optional wall mount.
So far I can only find them in Europe.
Does anyone know of their availability in North America or know of a similar competing product?