1. How hard is a DIY setup to connect up a aux battery.? Pretty easy
2. Is there a specific isolator that won't cost the earth to put in. Shop around, most would be around $100 mark
3. Been told 6B&S cable is a must. Yep
4. Can I get away with a sealed wet cell battery or again one that won't cost the earth but will also give good service. wet cell's aren't really sealed, but anyway yes you can use them
5. I currently have a 6mm wiring set up with Anderson plug to the back of the tug to run a caravan (done by the dealer) battery and it will also be running a 80amp calcium marine battery that is being installed in a new camper trailer. Will this effect how the Aux battery runs in the car. I should mention that the current cable doesn't have any isolator on it. Yes it will affect it, basically that cable is too small
6. is it possible to run the caravan/camper wiring thru theaux battery isolator?That is the most common way of doing it