Thanks for the response. Are there any ways to detect or narrow down the source of the leaks without getting into the spa cabinet? If I fill it up with water, let it sit, and find that the water level is lowered the next day, does that mean it's a shell leak for certain? or should I cover up the drain with something to make sure?
I've no idea how my old spa is constructed.. were they built very differently 15 years ago? I have a small access door on the outside of my three season room. When I open it, I see a motor, some electical, PVC pipes and some loose insulation. But that's just a small section that's completely surrounded by the cabinet, which does not look easy to get into, and I've no idea what I would find if I could get into it.