I’m assuming you did a through pump out and flush on the system. Could anything have been put into the tank during storage
If the tank is clean and because the smell presists.
1. Check for leaks at all the fitting connections.
2. Wipe down all the hoses and smell, especially those that are below the level of the tank top. The line sections below the tank top are the ones that will have had standing waste in them. Over time the smells will permeate the hoses. Three years is a short time for that to occur, but give it a try.
3. Do the same for all the hose fittings that are below the tank top. Contrary to earlier comments nylons, nylon can be a real cause of smells. Nylon is hydroscopic and absorbs much more moisture than any plastic used on a boat. Black fittings are most likely nylon. They’re either Marine
East of Forespar fittings. There are also very cheap
white (not PVC white) nylon fittings. Forespars nylon will absorb over 7% of its weight in moisture within several months, the other nylons probably over 8% in an even lesser time.
4. If the y-valves are Forespar valves (nylon) and are below the waterline, do the wipe test. PVC, ABS, polypropylene or polyethylene are quite moisture resistant. And should not be a cause of your smells, but can leak.
5. The tank is most likely polyethylene and shouldn’t smell. However. They are not overly strong and can crack at the welds and the fittings. I’d give the tank area a good looking over. If there is a crack, in addition to replacing the tank, I’d make sure the new tank well supported and locked down.
5. If your tank has a side mounted vent port. And if the line dips down or is flat for a distance waste material can build up there and clog the line. I’d give that a good checking over and check the line for smells.
6. I don’t know if the liming smells, but as someone mentioned, it can’t hurt to clear the line.
7. Again, as someone mentioned, the fittings may not have any teflon tape or paste and may leak.
8. The only other possibility I can think of is to see it any of the lines have a vented loop. If it’s stuck open it might be possibly the problem.
Hope that helps.