Yongkie,
We opted for the black water tank on the recommendation of the dealer. Apparently the outside storage compartment is deleted if you get the cassette toilet. On our unit, this compartment houses the wireless jack controller, and has tons of room (although you need a ladder or tall step-stool to access it while on the truck). Our 2000w Honda generator fits perfectly in the compartment with lots of room left over for extension cords, hoses, filters and whatever else you choose to put in there.