Twin electrics..

I have a p38 deisel with single point electrics.. but need twin electrics for the caravan.. I have a classic with twin electrics, how easy/difficult would it be to strip the charging feed out the classic and fit in the p38?

Wouldn't bother.
Run a 4mm.sq. fused supply from the + battery teminal to the rear l.h. corner of the car. Tucked away tou will find a spare plug with three wires...12V ignition live, Ground and Reverse lamp.
Buy a 20amp or higher double pole N/O & N/C relay, preferably with a mounting bracket attached. Use the 12V+ supply to feed both sets of contacts on the relay and use the 12V ignition live and ground wires on the spare plug to enrgise the relay coil. Connect the fridge and battery wires from your 12S (Grey) socket to the N/O contacts. Connect the 12V+ feed to the 12S for the caravan auxillaries to one of the N/C contacts.
The reverse lamp supply to the 12S socket is connected to the spare wire on the loom socket.
Whichever you decide, you have to run a wire from the battery to the rear of the car as there is no suitable spare live wire for the load required in the car.