50 or 30 amp

Now suppose you start with the case of  a dead short (low resistance fault) between your hot and neutral or between hot and ground conductors.   For example if someone accidentally drives a staple through a piece of Romex. (I'll admit it, I've done it).  Let's say this is a 15A circuit run with 14 AWG Romex. The resistance R between the wires and the staple is very low so current I is very high. Hundreds of amps probably.  Bad, right? No, not so bad, your circuit breaker will see a very high current and trip immediately. You'll have a dead circuit and a burnt up staple but the conductors won't have time to get hot so any damage will be localized.  Your electrical system is doing its job keeping you safe. 

Now, suppose instead of a staple driven through the wire you have a break in the insulation say caused by the cable abrading over time or by being pierced without direct metal to metal contact to the conductors. This might cause a parallel arc fault where in the electricity is flowing as a hot ionized gas discharge through the air between the conductors (line to neutral or line to ground).  We've all seen electrical arc faults probably. The resistance of that type of fault is much higher than a dead short  caused by a staple driven through the Romex, so the fault current is lower  . 

So now you might have say 10A flowing through that fault rather than hundreds of amps. Your circuit breaker won't trip until 15A is reached so this current will just continue to flow (for an indefinite period of time until something changes in the area of the fault). But, like in the case of the nail driven through the Romex, any damage is localized to the area of the fault. The conductors won't overheat because they are rated for 15A and only have 10A flowing through them. Your electrical system is still doing its job protecting you. 

If you now turn on that 10A hair dryer you will have 20A flowing through that circuit, 10A from the fault and 10A from the dryer.  Your conductors will begin to get warm but before long your circuit breaker will trip. Your electrical system is still doing its job. You'll be safe and now you have a hint that something is very wrong somewhere. 

This was exactly what happened to me on my outdoor circuit when I turned on my skillsaw and the breaker tripped, except that instead of an arc fault I had corrosion inside a receptacle which created a similar high resistance path for current to flow. If I had't tried to run the skillsaw I wouldn't have known the fault existed, at least not for awhile.

Now suppose you combine the high resistance fault in case 2 with a mistake by a previous homeowner where he installed a 20A breaker on that 15A circuit. You don't have a problem other than right where the fault is as long as you haven't turned on that hair dryer. Once you do you now have 20A flowing through those conductors which will heat up. But the cb won't trip to protect them. Not good, eventually a fire could start anywhere on that circuit run.

So back to our rPod case with the 50A receptacle feeding a circuit rated at 30A via the 50 to 30A dogbone. Now add on a high resistance fault from abraded wire or corrosion or something else, somewhere between the dogbone and the 30A breaker in the rPod panelboard. Let's say that high resistance fault is 10A. Now go turn on your a/c and microwave. You will have around 40A  flowing through those conductors and connectors which are rated for 30A. The 50A cb back at pedestal or wherever the park has put it won't trip, so those conductors will begin to overheat. 

Or, you could just have a "medium" resistance fault that allowed 40A to flow and the same thing would happen without any loads being on in the trailer. Or a 20A fault current with just the a/c turned on, etc. 

Those are the kind of scenarios which you are not protected from when you use that dogbone unless you add in a 30A circuit breaker like GlueGuy and I are suggesting. The 30A breaker in the rPod panel doesn't protect you in these cases because the fault is upstream of that point.  

Re StephenH's suggestion of using an arc fault breaker that would certainly add additional protection as it is intended to identify arc faults with currents below the trip rating of the conventional breaker. Because they can see arcs at lower current levels these can also help with lostagain's concern about the lamp cord, in fact that's the kind of scenario they were developed to protect against.

The last time I tried arc fault breakers was about 15 years ago when they were a new code requirement and they were really prone to nuisance tripping, I would imagine they are better by now. The AFCI's are code required for most dwelling outlet circuits which are 15-20A circuits so you might not be able to find them them with a 30A rating. You could look into putting them in the trailer panel though.  Its interesting that apparently they aren't yet required for RV use even though that's used as a dwelling.

To clarify when the NEC comes into play its whenever new work is done. So new construction but also if you have an electrician in to say replace your electrical panel he won't be able to do that without bringing that system up to current code.