changing old 120v wiring in 54 year old trailer

Window A/C that requires 30 amps? Must be a monster.
Most would go 240v before they go above 20 amp circuit.
120v 30a plug would be exceedingly rare.

Mark, First off you need to let us know a little bit about your trailer, how long, how many outlets and fixtures, are you adding a weathertite outlet box with a GFCI outlet. What about the fridge? Propane or 120 vac. You mentioned 12 vdc/120 vac fixtures, those are okay but a photo would show if they need rewiring. How many outlets and intended use. You've got the walls open, installing a few extra outlets won't hurt. I'd also look at RV wall switches for the light fixtures, beats the built in switches.

Is there a junction or service box with fuses in the trailer?

Here is a linkto a catalog where you can look up what is available. I think that assuming you'll have batteries and a 120 vac line you can find a nice little distribution panel. Are you going to want a converter/charger or just a charger. Converter/charger will provide 120 volts and 12 volts to charge the batts and light the fixtures.

12 vdc wiring can be stranded and for the most part #14 or #16 for light fixtures. The wires for the window AC will most likely be okay with #12 romex.

All in all it would be an easy job even for a novice.

If you want you can PM me and then we can post what we talk about later so all the guys are included.

You will want an AC on a dedicated circuit.

PS: this looks great....
http://www.bestconverter.com/PD4045-45-Amp-Inteli-Power-Mighty-Mini-Power-Center

Hey thanks...good thoughts, but you might be over my head some. I do want to put in a window AC at what would be the end of the circuit, hence the 10 gauge and 30 amps. Siding is off, ha ha, just a 1 by 4 skeleton at this point. There are currently some dual 12v/120v lighting fixtures I am going to try to re-use. So I would need a 30 amp breaker for the AC at the box, and that should be the single breaker for the otherwise not-too-complex circuit that runs through the trailer?

Assuming you are just going 30 amp service consider a new PD distribution box with built in 12v supply and both 120v breakers and 12v fuses all together.

http://www.bestconverter.com/PD4045-45-Amp-Inteli-Power-Mighty-Mini-Power-Center

Incoming wire from the 30 amp cord should be #10 and branch circuits with 20a breaker need to be #12 and any 15 amp breakers should feed #14 wire. Standard romex style wire is fine for 120 vac. You will have a 30 amp main breaker in the panel to attach the incoming wire from the 30 amp cord.

12v side... insert fuses to match the wire size. I would use stranded marine tinned wire for 12v connections.

https://www.genuinedealz.com

To do it, your first choir is going to be removing all the siding, either inside or outside, your choice. 12GA romex should be sufficient for everything but maybe the roof AC. Remember not much inside that oldie will be 120VAC. If there are 120VAC fixtures, your best bet is to replace them all with 12VDC fixtures. Same for the gas lights, take them out.