And you do have a clear plan of the basic layout dont you Sue
Slight adjustments can usually be made but a big change around can really throw a spanner in the works
So
You should have an idea where your 12v distribution point is going to be -you may already have a fuse board/breaker panel fitted which was/is used for the internal ambulance 12dc
Are you fitting a 240v battery charger or a specialised motorhome type charging /splitting/distribution unit. if so where can this be located,can it be close to the 12v distribution keeping all the electrical kit together-is there room for domestic battery how many batteries
Where is external hookup point going
MARK ALL THIS ON A SIMPLE LAYOUT PLAN stick about 6 bits of printer paper together and make the scale so its as big as possible
12vdc
Fridge
water pump
loo
heater
tv
12v sockets
Lights and switches
MARK ON PLAN
240
Fridge
tv socket
other sockets
MARK ON PLAN
Now then
Its not always necessary or desirable to run wiring behind vehicle linings but you obviously dont want it on show either
Armed with your plan you can start to work out how to run your wiring to each and every item
Just to complicate things a little further you have some maths to do
Because amps =watts divided by volts and 12 (volts) is a low number the amperage is high which in practice we need thicker cables than we might imagine
So we need to find the energy requirements of all the kit we are running wires to
I know this sounds boring but consider this
Your 2000 watt hairdryer at home on 240v =2000/240=8.3 amps
Your 200 watt hardly works 12v hairdryer 200/12= 16.6 amps
So we need to know the current consumption for each application
Then we have something called voltage drop
This means that for long cable runs we have to increase the thickness of cables
Here is a really easy to read explanation and all the formulas and charts
ME03
But we are going to cheat a bit here cos when you post all the current requirements of all your kit 10 people are going to do the maths for you
more shortly