As far as the the tank surface smelling of H2S, I don't think they permeate. I think you're looking at a transfer of smell from one or more bad hoses.
I agree with Ralph.
As far as the sensor flange leaking, the factory sealant is 3M 4200 which doesn't stick to PE so hence the leakage. You can remove the flange, clean it up and use Permatex #2 which is about the only sealant which will stick to PE.
Better yet, don't use any sealant...instead use a rubber gasket to seal the flange and a rubber o-ring to seal the cap. Both are available from Beckson and other inspection port mfrs.
As for a new tank, DuraCast standard wall thickness is only 1/4". Ronco Plastics
Ronco Plastics marine Tanks is your best source for a new tank. They make TOP quality thick-walled (3/8") water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangula.. And they install fittings in the sizes and locations specified by the customer when they make the tank.
And finally...the tank, unless it's leaking is never the source of odor INSIDE the boat because odor from inside the tank has only one place to go: out the tank vent, gassing everyone within as much as 50 yards of the boat.
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--Peggie