The mechanic was right, the tanks on these things are many times thicker than the portable cylinders. You can remove the tank when you run it empty, use a wire brush to clean it and paint it to stop any more corrosion.
You may be able to have it inspected at places that do fire extinguisher and other tank certifications if you really need the peace of mind. They use a hydro test process on portable tanks.
1998 bounder 36s V10 F53