Any metal is a bad choice for sewage holding
Urine is so corrosive that it typically eats through a weld at a seam or a fitting in 2-5 years...and will turn the sheet metal--whether it's aluminum or steel--into a collander within 10 years...sooner with aluminum. The ONLY material that should be used for sewage holding is thick-walled (at least 3/8") polyethylene. Good for at least 25 years.
