I don't know about the v3 but I do know about the r4. It's Radio Shack's adaptaplug M. Make sure the "tip" (the inside of the plug) is negative. The "ring" (outside) is positive. I assume your SLA is 12 volts. That will power the r4 without any problem.
Shouldn't harm anything to power both the v3 and r4 at the same time. It'll just drain your battery a little faster.
As for the wiring, the connection on the battery side is probably going to depend on what you've got to work with on the battery. I would just connect the wire and the plugs for the r4 and v3 permanently to each battery. You could put something in the middle to connect/disconnect the wires for the r4/v3 but that's just another point of failure if you ask me.
Neither of these units draw much current. So, the wire gauge doesn't need to be very big, unless the wire is very long. Just about any wire you'll buy will be fine. Get some flexible, stranded wire. The wire Radio Shack sells to connect to their adaptaplugs will work well.
Edit to add: Having a fuse in this is a very good idea. Throw an extra fuse or two in your gear bag too.