These is a fuse for the battery connect relay controller in the battery control center, which is probably beside your steps. If the fuse is okay but you can't get the emergency start to work you could hot wire it.
I don't see with my cell phone app your year model. FR has changed the BCC since ours was built as a 2011 model, but yours may be similar. If you have enough charge in your coach batteries, unplug the connector from the black Intellectec BIRD and run a jumper from the Coach Battery terminal on the panel to the relay terminal with the red wire. You could get someone to do this while you start the engine, or vice versa. You should hear the relay click when you touch the wire to the terminal.
Maybe you could just run a jumper cable from the coach batteries' positive terminal to the chassis battery. You only need the positive cable. They are already grounded together.
Another way is to jump on the back side of the battery control center. Get underneath and look behind the BCC. You'll see some red cables. One is on a terminal marked "Chassis Battery" and one is marked "Coach Battery". Jump across them. Wear face protection- you may get a spark.
2011 Sunseeker 3100
1997 Pathfinder Toad