If you want a breaker to fit the Zinsco box, Challenger may still make them. Pricey. Zinsco on an electrical box is like Yugo on an automobile.
From the 50 amp breaker, all conductors from there until the next device or panel must be suitable for 50 amps.
I suspect that the RV really needs 120/240 volt power which would require 10/3 with ground for 30 amps. And that cable must be protected at 30 amperes.
Additional ground rod not required.

A 30A RV circuit is a 120V/30A.
A 50A RV circuit is 120/240V/50A.

50 feet is FINE to keep with 10/2.

You certainly can still get Zinsco breakers. Changing the panel is your best bet, but barring that you can still get breakers. Try a real electrical supply house, or maybe the interwebs. Oh, and no need for a GFI breaker either.

http://www.homedepot.com/b/Electrical-Power-Distribution-Circuit-Breakers/Zinsco/N-5yc1vZbm16Z50f

Here is a link that might be useful: Zinsco breakers at Home Depot

This post was edited by petey_racer on Wed, Aug 13, 14 at 18:55

Zinsco breakers are sometimes also called Siemens. They are available though as pointed out can be pricey since they aren't the commodity item like breakers for other more popular panels.

Yes, you can feed a subpanel with the hot tub breaker. You run a ground from the subpanel (along with the grounded conductor, aka neutral). 10 g is NOT big enough to be protected by a 50A breaker either to the subpanel or to the RV receptacle.

I found a source for 30 amp single pole breakers for my box so I think I will just pull that 50 amp out, and run 10/2 from the panel to the garage. My understanding is that as long as the wire is up and running through holes in the joists that I don't need conduit. I plan to penetrate the rim joist and mount the power box on the outside. Do I need some kind of bulkhead fitting where the wire enters the backside of the box, or can I run it straight in?

Thanks everyone for their suggestions!

You need a fitting appropriate for the whatever is coming into the box (I presume type NM). Conduit is not strictly required but if there's a chance of physical damage it's permitted.

OP, Is this garage a separate building? If so, does it have power to it now?

Others, If the answer is yes to both, does this violate the rules requiring only one circuit to a separate building?

If is is a separate structure he has all sorts of issues. I assumed when he was talking about mounting a box on his rim joint he was talking about moving a circuit within the same structure.

Seperate or detatched garage is somthing the op needs to clear up`before asking any more questions. I also don't understand why op has a "rim joist" in his garage period,attached or detattched unless it's two story and outlet is to be 8+feet above ground. He clearly states "I plan to penetrate the rim joist and mount the power box on the outside."

Zinsco/Sylvania breaker are still available. Be forewarned of sticker shock.

AZ Partsmaster
Midland Hardware
Sasan Electricals

All sell them.

A friend replaced a breaker box due to it being a Zinsco/Sylvania and breakers were so hard and expensive to get.

They aren't hard. I was even able to get 60A breakers for it. It just was expensive. That coupled that some of the older breakers in it were failing, I replaced it. I made things very happy for some of my neighbors who still had the panels when I put mine up for grabs at the neighborhood yard sale.

I'm probably using the wrong term for the joist, or whatever it is called. Hopefully, this description helps.

My garage is attached. I have a raised ranch home. I'm talking about penetrating the joist that sits on top of the concrete foundation wall and mounting the RV outlet box on the outside of the house.

So I found a box by Connecticut Electric that has a 30amp and a 20amp breaker, that would take a 50 amp 240v feed. The 30amp being on one pole and the 20amp on the other. I'm thinking I might just run four six 6 gauge wire to it and be done.

The box sounds like it must be a sub panel. If that is so, you might want to consider adding a more standard type panel in the garage so you can add some circuits to the inside of the garage if you need them some day.

I think ionized that he's referring to one of these combo panel / RV shore-line receptacles.

The question is, why a subpanel at all. If it's just feeding a single hookup receptacle, there's no purpose in putting a subpanel there.