I understand what you're trying to accomplish. Why waste all that space in the sink tank. Here's what I would do.
It's going to depend somewhat on the location of the two tanks and access between them. I would use 1 1/2" or 2" PVC pipe. You will need connections, pipe cleaner and glue. Hardest part will be making a good connection between the tank and the pipe. Ideal would some type of gasket system, but for that you would need access to the inside of the tank. You may can use plumbers putty and glue but it would have to be a very secure connection to keep from vibrating loose driving down the road. Go to a plumbing supply store (not a big box store a real plumbing store) and tell them what you have in mind. I'm sure there is something out there that will work. Also make sure all the pipe is secured to frame to decrease vibration when traveling.
Install the pipe toward the TOP of the tanks. As the tank that's used most often fills it will begin to siphon over to the other tank. Problem solved.