Ok just had a look to see what unit I have in the van and it is a ST20 11. So now for the silly questions I noticed what looks like a blade fuse that was marked Batt, is this just a cover for where I need to connect the batt to or is it just a fuse
You will need to gain access to the rear of your St20 to connect the wires, depending on your expanda model you may have to take out a few screws to get at it. Some models have them so as you can get to the wiring without removing the st20, on my 14 it is mounted below the fridge so have to pull it out to get access. Yes the blade fuse marked Batt is the main fuse that connects to your battery, on your ST20 it should be a yellow 20A fuse. If you don't have a fuse in it you will have to fit one when you install the battery. Have a look at the block diagram bellow to get an idea of how it all connects.

As you can see the positive connection goes through a 20A inline fuse then on to the main Battery switch and finally connects to the "BAT +VE" terminal on your ST20. The inline fuse should be as close to the battery as you can get it but not inside the battery box. The negative wire connects strait from the battery negative to the "BAT -VE" terminal on the ST20.
If you have to remove the ST20.....
First make sure you don't have 240v power connected to the van.
Remove the one screw where the fuses are located. Then remove the complete plastic front panel. It is held on by six clips, three on the top and three on the bottom. The easiest way is to push up on the bottom of the panel with you hands until it unclips at the top and then move it down to detach the bottom clips. Take care you don't bend the blade fuses when removing the panel.

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Whith the front panel removed you now just have to remove the four corner screws and slide the ST20 out. The ST20 is quite heavy so have an old towel or similar to rest it on so as not to damage the floor. You can see clearly in the last pic where the wires will connect to. For the ST20 6mm twin sheathed cable should be fine for the short distance you will need to run. In the pic above right you can see the main battery switch that Jayco use.

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You can see you must use covered crimp terminals to connect to the ST20 as there isn't a lot of room where they connect in. you will also need a set of crimp pliers to make sure you get a good connection.