Godzilla41:
Also when parked up, with phone charger, portable dvd player and possibly electric coolbox going, will I need to keep my engine running at all so not too drain the batteries?
There’s no real need…
To charge yer phone and DVD player up overnight if you leave them plugged in all day. If you’re worried about a flat battery in the morning then, charge everything during the day whilst yer engine is revving happily and leave the minimum amount of gadgets plugged in overnight.