Venting isn't the only concern when it comes down to oders. Given enough time the tank itself begins to smell funny. Also, input connections like the slip fittings for tanks work well, seal 'fairly well' too, but if you fill that tank and not empty it before realizing it's full, you'll have sewage spewing out the connectors underneath your bed.
Having full times in an RV for 3.5 years, and in my bus now for 6 months I can clue you in one other reason you don't want to do this. On a windy day you are going to notice that wind pressure on the outside will actually force air into the tanks via the vent pipes, and air under pressure will find a place to escape. I've noticed on some windy days a whiff or two of something not so pleasant in the bathroom, especially when flushing as you get a blast of stinky air blow up right in your face.
Personally I wouldn't consider this in any manner, even more so underneath the bed.
If you look at the way professionally built RV's made, they all have the gray and black tanks underneath the unit. There must be a reason they do it this way!!