Yeah I know these tanks are heavy enough, but I pull a boat and I`m buying an electric winch. I`d rather have a second battery, always charged, by the system when I need to pull the boat up the trailer.
Other option was putting a battery on the trailer itself, but it wouldn`t be always charged. And it looks fugley!
Or, place a battery in back of trunk. But won`t be charged either, and would have to "strap it down" I guess with a battery box.

Any other ideas ?

i would get a second battery and put a battery isolater on it, what it will do is charge the battery that needs to be charged that way you can run the winch and the mada with no problems

Yeah, but I don`t see a spot to mount it. Actually, the battery can be wired pos to pos/neg to neg, as long as the original battery is fairly new. This way, it won`t draw down the original.
I did this to my 96 Ram that I used to plow with. On PlowSite.com, most of the plowers that need to run the plow and lights, wire it this way.