Original breaker box? 1964 Shasta Compact

The old box is probably fine and most of the original wiring. Just inspect for damage and scorch marks. If it's OK, no reason to replace it. The trailer was originally 15 amp total. You probably have one 15 amp breaker in the box.

Please post a photo of the box. I'm guessing you can use it as they rarely are damaged. Also PM me for my email address. I will be happy to share lots of photos with you. (too many to post here)

Quick notes on Breaker Box wiring:
1. Main comes into the box from the exterior inlet. In your case, pry 14/2 15 amp. I upgrade to 10/2 main.
2. All copper grounds go on one bar. (sometimes the main ground to the frame is aluminum) These bare copper grounds are not on the same bar as white neutrals.
3. All white neutrals are together on another bar which is isolated from the metal box. (bakelite/plastic/non conductive material) In other words, the neutrals NEVER contact the trailer grounds.
4.The black hot wire normally connects to the single breaker in the box; sometimes, it connects to a hot lug. That depends on the box.

Very few trailers had a main breaker. Instead they had a single breaker running the entire trailer and relied on the campground system for a main.

Again, you're welcome to PM me for an email address.