I just cleaned my freshwater tank on my 1996 Vista. I pumped all the water out then put the bleach mixture (1/4 cup Clorox with 1 gallon of water per 15 gallons of tank) in for 3 hours..and then filled the tank and flushed that through all of the taps, sink, shower, and transom washdown, and re-filled. According to the manual that's all I should have to do.
If you really need to get that water out of the bottom I suppose you could try to siphon it out.
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Chris and Alicia
Las Vegas, NV
ALTIVA
1996 Four Winns 238 Vista Dlx 5.8Fi