I'm considering this option for charging the break away - is it really just as simple as connecting +ve to +ve and -ve to -ve on the 2 batteries? What size fuses and diode would I need to use? It seems a lot simpler than installing solar regulator and interfering with the solar setup.
Pretty much yes that is all you need to do. As you are leaving the breakaway battery in place you will only need to fit small fuses to allow the battery to be kept topped up.
If you use something small say 2 to 3 amp fuses it will be ok. The added advantage of using small fuses is that once the breakaway is triggered it will fairly quickly drain the breakaway battery then the small fuses will be unable to supply the needed current so they will blow and prevent your van battery from being drawn down as well. This would be a good safeguard against an accidental activation.
You will need to make sure the breakaway battery is close to fully charged when you first hook it up or it may keep blowing the fuses.
As for the diode any good sized power diode will work ok. You should get one that is rated at least 3 times the size of the fuse you use. standard silicone power diodes will be ok but they will give you a voltage drop of around .6v if you can you will be better with the same type of diode they use in solar panels. these are called Schottky diodes and will usually only drop the voltage by .2v . If you are handy with a soldering iron something like
this will be a good choice. Be aware that on that diode the metal tab with the mounting hole will also be live once you connect the power so you will need to keep it isolated from the chassis etc. perhaps cover it in heatshrink and tape it into your wiring loom.
