I had the whole belly wrap(coroplast)down on mine and you can see up to the floor that way. Where ever the holding tanks are, the width of that amount has no cross brace in the frame for the 2x2 + 3/4" plywood to rest on. Since you said it was at/near the bathroom, I'm thinking this is that span. Mine is not as sturdy in those areas either. I've heard a creak or two around those areas too. Between the tanks they put an abbreviated cross member that spans the I-beam frame to help stabilize the floor. By abbreviated I mean a standard structural cross member is for all practical purposes the same height as the frame rails. If the frame rail is 6" I-beam the cross member is 5-3/4". An abbreviated cross member is 3" so the black and gray water tanks can be linked together to a common sewer dump location but each with their own slide valve. All this to say.....yes it is going to be a little spongy there and you may get a creak or two but nothing to be alarmed about.........
2015 Creek Side 20fq, 450 Watts of Samlex Solar,
2013 GMC Denali, Duramax, Retired in 2021