Thats what I figured after much thinking and looking last night.
I've already cut an access door out in my closet to get to the trap and drain (hope they arnt too seized up!) and I'm getting ready to remove some more of the lower drywall so it can slide out.
fortunatly, looking under the tub from my little access door... the floor under the tub looks very good! Its from the tub forward that is rotted to dirt... I guess from years of people not closing the shower curtain all the way.