I would like your opinions on selecting a fresh water holding tank for my 2008 Advantage 29 foot mid-cabin X-Flight. Because it was originally a fresh water boat the water supply for the front hand shower got unlimited fresh water from a hose that ran to the bottom of the boat. Because it is now going to be a salt water boat I have sealed up this hole in the bottom of the boat. There also is a small sink in the mid-cabin that gets its water supply from only a small 2.5 gallon holding tank located on the side of the engine compartment. Both fresh water connections have their own dedicated 12 volt pumps. What I would like to do is change out the small 2.5 gallon holding tank with a larger one that would supply fresh water for both the sink and front hand shower. Because I have plenty of room in my engine compartment what size fresh water tank would be ample enough for my fresh water supply?

Take some measurements and go to the RV dealer. They have hundreds of options available for fresh water holding tanks, but I would say that 10-15 gallons would be more than plenty.

Interesting you mention an RV dealer. In Tampa, at our Fairgrounds, they have a show going on through the weekend with many vendors. I think I will pay them a visit. Thanks for the advice.