All work must conform to state and local building codes. A fuel tank can tip over or float in a flood, causing fuel to spill or catch fire. Cleaning up a house that has been inundated with flood waters containing fuel oil can be extremely difficult and costly. Fuel tanks should be securely anchored to the floor. Make sure vents and fill line openings are above projected flood levels.
Propane tanks are the property of the propane company.
You'll need written permission to anchor them. Ask whether the company can do it first. Be sure all work conforms to state and local building codes. Install a floating floor drain plug at the current drain location. If the floor drain pipe backs up, the float will rise and plug the drain.