Unfortunately, many RVs simply do not provide any power sources for owner-installed goodies, including my RV. Now, anyone reading my website already knows I generally don't install stuff just to install it, but rather to add a desired functionality or to improve something that is lacking in the unit.
That is the situation I find myself in. I want to add a Tire Pressure Monitoring System (TPMS) to the rig, as it is very important with any RV to monitor your tires. Blow-outs are all too common (due to cheap tires, cheap suspensions, owner neglect, and poor driving habits), and no one wants to encounter a driving condition from a blow-out.
Like most trailers, a TPMS repeater/booster is often required to obtain sufficient signal strength from the tire sensors to the cab of the tow vehicle. And adding a repeater means wiring it to 12V.
As well, we would like to put an additional LED light strip on the nose of the RV. This I suppose could be considered bling rather than something necessary, however it sure makes things a lot easier at night if you are in a darkly lit camping location. And additional lighting makes it nice when you are having to hitch the RV at night.
For these reasons and future additions, I found it necessary to add a 12V distribution panel to the front utility/storage compartment.
A detailed explaination of this characteristic can be found here: High Power DC Wiring Principles.
Otherwise, the chart below will be sufficient to size the wiring needed:
Note that the chart is not the absolute maximum current capacity of the wiring, but rather the maximum current allowed that will result in a 3% or less voltage drop over a given distance (which is always less than the wire's maximum current carrying capability).
Since I will be tapping into the RV's main disconnect switch, I will need about 10ft of wire to connect the distribution panel (you have to account for both wires; the positive and negative side). The chart shows that to limit the voltage drop to 3% maximum (approx 0.4V max), using 8AWG wire will provide up to 50A of service. And as marine grade (105DegC insulation) rated 10AWG could carry the 50A load, you can see that we have to go to 8AWG due to the voltage drop consideration, not the current carrying capability.
Since I will be using a 6 circuit fused distribution panel, I can create six 8Amp circuits, or any other combination does not exceed 50A. Terminals for 8AWG wire can sometimes be hard to find, but I was able to find most of what I needed locally at the big-box auto supply stores. The few items I could not find locally I found on-line. I live in a small town (about 60,000 people) so I can't always find what I need locally. My rule is if I cannot find what I need after looking in 3 stores - I'm buying it on-line.
While I am always mindful that my RV is a "lightweight" unit, and I have to really watch the weight (my cargo carrying capacity is limited), this project only added about 3lbs of weight to the rig.