Those custom-sized tanks are damned expensive. I had the same symptoms; I could smell the leak every now and then but nothing showed on the soap test. Out it all came. I replaced the ten-pounder in the locker with a twenty hanging off the rail. It isn't marine-grade but you exchange it when empty so that isn't a problem.
As others have mentioned make sure you aren't mixing pipe-thread fittings(which must be sealed with PROPER gas tape-don't use plumbing tape) with compression fittings (which must NOT be sealed with tape) or you will have a leak. Also my regulator had to be mounted with the output facing down.