We have the 2020 39RKFB Legacy and had similar concerns with the fridge. Our solution, BALLOONS! Picked up a dozen of quality 12 inch and 24 inch latex balloons, blew them up half way and placed in between the items and the shelf above or in front and have had great success. By only filling halfway they can squeeze in between some of the odd shapes. Worked so good that we now use them for securing all of our storage ( dishes, pantry etc ) on travel days.
As for bouncing, it does tend to get bad on rough roads but the air ride hitch absorbs a lot.
As for the drain, haven’t noticed any issues but I do primarily use the half bath for seated operations
. However, it has 2 black tanks so it really doesn’t have that long of a run.
Overall, the missus and I love the floor plan and it works great for us.
Good luck on finding the right rig!