I tend to keep it simple. For starters, your trailer brake, turn, running, and marker lights are all 12 volt and are powered from your tow vehicle. So standard trailer light
wiring works for that. Go LED if you can, they can (if built right) last longer and use less power.
For your internal lights and water pump (and a heater fan?) you will need a 12 volt deep cycle battery. A fuse or breaker panel so you can protect against shorts causing fires is important. Put a fuse on each circuit. One for the internal lights, one for the water pump, etc.
One of those circuits can be for an inverter so you can get 120 volts AC for household appliances. I don't normally bother with this, but it can be helpful to charge cell phones and small things like that. Don't plan on heating or cooking off an inverter. They are not that efficient in the first place, and your battery does not have that much power compared to cooking or heating needs. Plan on Propane
for cooking and heating, or AC power from a full hook up camp site.
As far as solar
panels go, they can charge that deep cycle battery. The more watts the better, but you can go overboard. You might want to do a "power budget" if you plan on boondocking
for extended periods of time.
An alternative to solar cells, or a supplement, if you are away from AC power for extended periods of time, is a small generator
. They make quiet ones now and the energy in a gallon of gas is more than in a large deep cycle battery.
Food for thought.