I have a big tub (2 people size) and no way to access that elbow at
the drain flange drain pipe unless I cut the bottom of the modular
home.

When we lived in our mobile home, there was a panel under the floor that
provided access to the tub drain. It was simply a 2'x2' piece of plywood
screwed in place.

If you have something similar, it should be fairly easy to climb underneath
and have a look.

Anthony