Sorry, no advice on your plumbing. But we have had spas since 1992, and got a new Coleman in 2004. This last October, 2005, we had an Eco-Spa salt generation unit installed which is made by Balboa. You can do a web search for info on this. It uses a couple of cups full of ordinary solar salt in your newly changed spa water to extract the chlorine from the salt to disinfect your spa water and recycyles it. You do not need to add any bromine or chlorine to your water to keep it chrystal clear with no irritants on your skin. Only check it to balance PH. Works beautifully! We have never had the water look, smell or feel better! Costs about $1000 or less installed; less if you can cut the lines and intall it yourself. Will pay for itself with savings on chemicals. Try it; you'll like it!
By the way, have you tried gluing the coupler with PVC cement?