Hi @RudiRudiRudi,
Split charge relay is fine and loads cheaper. Euro 6+ need the expensive stuff.
Durite works for me and I just got a kit from Simply Split charge to make life simple.
https://www.simplysplitcharge.co.uk/index.php?route=product/category&path=109_98
The owner will make up any kit that you want from his set versions. Have a chat about your needs.
I ended up with about 7m of Red cable required. Medium duty kit for a 110Ah battery. I added a fuse box but that is only needed if you have multiple feeds/outlets. Easiest to get it all made up for you to save loads of hassle ... Heavy duty crimping and/or a large soldering iron needed otherwise.
Comes with simple, easy to follow instructions.
No spare wheel - Just a "repair" kit in my Roomster, so easy to just remove the polystyrene and get loads of space for the battery and whatever you need storing.
HTH - Have fun...