On my Coachmen (a '98 Santara 315QB), under the bed is the fresh water tank, some wiring, the water pump, and a little free space. The easiest way to get at those things is to remove the mattress from the bed and unscrew the OSB panel that forms the top of the bed itself and remove it. There was enough available space in there that I could (and did) add a handy little electronic safe, which I know is not very secure but does provide a convenient place to keep passports, petty cash, chocolate bars reserved for s'mores, etc.
Under the bathroom floor I'd guess is the black and/or gray water tanks. I could be all wrong there. There does need to be some vertical space under the shower pan for the trap for its drain, but that doesn't apply to the toilet or bathroom sink (at least in terms of floor height).