Is 12v constant from hitch actually necessary?

smkettner wrote:
If you start fully charged and always arrive at the destination with power to plug in then no go ahead and pull the fuse.

If and ONLY if you know that your battery (ies) will not discharge below voltage needed to apply brakes to your rig if it should become detached. How is one to know if the batteries are being discharged below the safe level while driving without charging thus shortening their life?