I have the same problem on my LaSabre. I put an ampmeter in series with the battery and cabel, I started pulling fuses but could not locate any one or two that would stop the current drain. Th Buick dealer told me that the computer sometimes backfeeds when fuses are pulled. I installed blade switch like you did. My battery is located under the back seat and it is a pain to get at all the time.
So I installed a battery charge circuit from the M/H to the toad and I no longer have to worry about cutting off the battery and the car is always ready to roll.
If you have any additional questions, please feel free to ask.
Don & Bev Morgan Weyauwega WI, 05 Itasca Horizon 40KD, 400 HP Cummins, Adopted by a great couple, Toad 07 Saturn Vue AWD, Air Force One, TST 510 TPMS, Mayor of Weyauwega 2007 - 2013, Waupaca Co Board Supervisor 2010 - 2014