When was the last time the hoses were replaced on the stbd tank? No mater how "impervious" the hoses claim to be, eventually they break down with no visual clues. Contrary to popular opinion, I replaced all my blackwater lines with sched 40 pvc 2 years ago with no leaks
, no smells, no problems...
Also might want to check your vent lines on the stbd tank. If you have a fancy charcoal filter might be time to replace the charge. Can get activated charcoal at the local aquarium supply much cheaper than a boat supply house.