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I haven't seen it mentioned BUT there is an after market company that makes them for this space so you can do it if you want to. Made by Areotank. Here is link
It wont be hard to hook it up on a toggle switch to swap tanks. I have my axualliary tank hooked up this way. I even have a sending unit in the tank that reads on my factory gauge. I used an install kit from http://kshmarine.com It basicaly is a splitter valve. The kit was about 250. I found the same valve in a JC Whitney book just the other day for $75. Mine works great I have a 91 gal. tool box tank combo from rds. If any one is interested I can post the detailed instructions.