Holding tank or Fuel for Jeanneau
Unless you use one of ''modern'' very costly 2000 series of stainless you are going to be VERY dissatisfied after you use a 300 series of stainless as the material for a sewage tank. There is NO way that you cannot avoid welds and the corrosion into these ''heat affected zones'' plus the extreme attack by acids and products of fermentation will make your tank a ''swiss cheese'' especially along the weld lines ... in a very short time. Even if you grind all the welds smooth and electropolish and ''passivate'' the internal surfaces. The correct metal for such an application is hastalloy, etc. .... and that cost would be absolutely prohibitive. Sewage fermentation (an anaerobic process) in a tank consumes all the available oxygen (needed to keep stainless stainless) and unless you also plan to ''sparge'' air into the tank continually, eventually ALL wetted weld areas will fail due to an accelerated ''crevice corrosion''. Stainless is a BAD idea for such a tank that isnt continually injected/pumped with air/oxygen ... as would a standard fermentation vessel.
Polyethylene (or polyethyelene prepegged to a fiberglass backing panel) is about the only way to go. Polyethylene of normal structural thickness doesnt have the ''intersticies'' in its macro-molecular structure to permit stink transmission (vapor pressure pathways) and is quite suitable. You can buy sheets of fiberglass panels already pre-bonded with polyethylene if you want to make your own poly-lined FRG tank ... if you know how to weld polyethylene at the seam lines.
If you have already built a stainless tank for such a purpose, be absolutely sure that you apply a very LARGE cross section atmospheric vent to assure an aerobic fermentation and a vent large enough so that the external ''wind'' can help sweep out the heavier than air ''blanketing gases'' that naturally form.