Propane tanks in the SL and SD models are horizontal tanks, and distinctly different from the vertical tanks used on other TM models. They are NOT interchangeable.
If your propane supplier won't fill your tank because it's too old, you may just need to get the tank recertified. Large suppliers like Amerigas will do this for a nominal fee ($10-$20) -- they sometimes remove the valve, do a hydrostatic test, take a peak inside, and then stamp a new date on the tank, which will allow fillers to legally fill the tank. Sometimes, they just stamp the new date.
If you need new tanks, be prepared to spend ~$150-$200 each. I believe I've posted here where I bought mine, as I cannot remember where.
Dave