I'm getting ready to do something similar. I have a toy hauler TT but the water tank is just behind the tongue so it is very tongue heavy when the back is light and I have to take in water. I got a second FW tank I am going to mount under the back with galvanized metal strap, probably 6 or 8 straps. It will have its own filler in the garage near the access on the side and then I want to route the water forward to the original tank for use. Running the line forward to the other tank seems like it will be the hard part.
Also, I'm going to put an inline UV filter and particle filter between the two so as the water is pumped from the rear to the front it is more or less sterilized. Not very expensive and it certainly can't hurt.
I'd love to see pics on how yours turns out.