Fresh water tank
What you think is sediment on your tank may be algae. Since my tank is under the camper floor, exposed to daylight, and white translucent polyethylene, I had alot of algae growth.
As for the plugs in your tank, if they are sealed, and are not drain valves I wouldn't touch them...my tank has two on the top of the tank which I believe are from the rotomolding process of making the tank, and they are sealed.
The bent tank support you mention...I would first try to figure out what caused it to bend. Perhaps not heavy duty enough to support the tank weight. My tank supports are bolted to the frame, if your bent one is...perhaps a welding shopcan fabricate a new stronger one using the bent one as a template.