What I would do first is make up a diagram of the electrical lay out of the trailer. Actually this will be 2 diagrams, one for 110 volt items and one for the 12 volt
items. Sketch in the location of each of the 110 volt item and it's load rating (wattage). This will allow you to sketch in the wiring as it works back to the 110 volt panel. You can group general 110 receptacles together. Plan on GFI receptacles for the bath room and close to the kitchen sink. If you use an electric cook top and hot water heater they will each require a separate circuit. Also figure on a circuit for the converter to 12 volts.
Once you have designed the 110 circuity that will dictate what size panel you need and can get at any electrical supply house.
Now do the same thing for the 12 volt
lighting. You can get a 12 panel at most RV shops.
I really question why you are not considering propane a it would greatly reduce your 110 load, the stove and hot water heater , and greatly increase the places you could camp.