I have owned 3 Kingscrafts and just simply love them. The water tanks are part of the keel as you mentioned. I would not bet all my $ on it being 125 gallons since they welded these boats and it all varies, or the pick-up tube actually reaching the bottom. Cutting an inspection hole should be easy enough. Just make a plate with 4 holes and tap the water tank top to mount. Personally on our current 55' Kings, I shut off the keel tank and mounted 4, 25 gallon plastic tanks in series. 100 gallons is plenty for us, and it has no metallic taste. Also have to watch these original keel tanks for corroding through. Then you will have an eternal supply of water, albeit, not so fresh.