120v Electrical for my trailer?

I just bought a 1964 Aloha camper trailer, 13 footer.  My plan is to gut it and re-make it into a home office in my back yard. There are remnants of an electrical system there now, none of it is completely hooked up.  I'm not worried about the electrical for the break lights and turn signals - I can get a kit to wire those up to the plug that then would plug into the back of my truck for towing. No problem, especially since I'm tearing out all the paneling.  My question is about the 120V wiring.  It appears that the system was wired like this:

1. It had an inlet on the exterior of the camper like this: http://www.bigwoodyteardropcampers.com/male-power-inlet-15a/ that one would plug in an extension cord, presumably then plugged into the campsite power source (or in my case, a standard outlet on the back of my house).

2. The inlet was then wired up to a breaker box inside a cabinet. There's just one switch on that old breaker box, because it's all just a 15 amp system. I don't know if the breaker box is functional or not.

3. From that breaker box, the wires went to a couple of standard wall outlets and a couple of wall lights.

My questions: First, do I really need that breaker box inside the camper?  The extension cord I'd hook up to this camper will be plugged into a GFCI protected circuit on the back of my house, which is of course wired through my home circuit breaker.

Second, if I do need that breaker box for some reason, anyone have a link to where I can get a new one, assuming the one I have there that is nearly 50 years old isn't totally safe anymore? I don't need a huge fancy box for a 50 amp connection to a big RV. This is a very small camper that will power my laptop and a light, and possibly a very small space heater (think 400 watts or so), and that's it.

Thoughts from the experts here?  Thanks in advance!

Since you won't have many circuits, you won't need a box. Get a good extension cord, not the cheap kind. How far will you be away from your home?

Could be 50 feet, maybe a little less.  But yeah, just the one circuit - I'm thinking from the power inlet, I'll just run romex straight to two power outlets, one on each end (so I can choose which side to sit on with my laptop, depending on which window I want to look out of), and that will be it. I probably won't even hard-wire any lights at all - I'll just put a nice lamp in there, and it will feel like a nice little home office. I can't imagine needing another circuit breaker for just two outlets.

I do figure on just getting a really, really good 50' extension cord. It's a one-time investment, and I don't want that to be the problem down the road.

I know this post is old but as far as breaker box..it is required in mobile homes and rvs per the national electric code.  its up to you if you put it in or not..onto teh cord..For safety..you say im going to get a good one??  cords are rated by amperage and usage type.  Again quoting National Electric code..there is cord rated for rv use.  what ever cord u buy, make sure it can handle the amperage you intend to pull through it.  Id assume ur load is about 20 amps or less..so you would be good with 12 gauge wire.