Our old motorhome had the big bathtub/shower in it with a awfully soft floor in it. Every time I stepped in it, It creaked and cracked like I was going to fall thru the bottom.
I had a small access to it, but there was enough room to build a cardboard wall around the bottom of the tub floor - and then used the expanding foam inside of that space to fill up the cavity between the tub and floor. It worked pretty well for many years.
If I were to do it again, I would put a trash can liner in the area, and fill the foam inside of the trash liner.
Steve & Stacy,
Oliver, Ned and Ellie ( Yellow Lab, Red Lab, Border Collie/Boxer)
2015 Seneca 36FK
2007 Toyota FJC - on the trailer