10 to 20 gallon is way tooo  small. waste has to be 20% more than fresh water tank. google ‘parting rv’ or find someone who has a large auto salvage junkyard as they often have junk rvs you can get both fresh water, grey water and 12v water pumps from for way cheap. also water heaters awnings and more. youre gonna need 40 50 gallon which are flatter and dont take up much room. good luck and keep us posted.