I think you might be able to find some 50 amp plug to dual 30 amp socket adapters. I assume they are made with some sort of 30 amp in-line fuses or they would not be safe and likely would not be sold by anyone for legal liability reasons. The L1 of the 50 amp service would feed one 30 amp socket and the L2 would feed the other, each would have a 30 amp fuse or circuit breaker in series with it.
Why would you need that for RV's?
Everything we deal with here is either a single 30 or single 50 amp feeding the RV, so either a single 30 or 50 is all that is needed.
There is an adapter available that will allow a 50 amp coach to get power from two 30 amp receptacles or 0ne 30 amp and one 20 amp, but it requires two separate circuits to work (not a 50 amp split into two 30's at the pedestal, and no GFI).
Here is one example.
http://www.campingworld.com/shopping/item/50-amp-rv-box-adapter/25774#sthash.teqsIgJX.dpbs
These adapters will not work with a GFI outlet and you still have limited power coming in. Instead of 100 amps (50+50) you have either 60 amps (30+30) or 45-50 amps (30+15 or 30+20).
If it works you will have 40-50 amps less power available then with a full 50 hook up, but you may have enough to run just about everything everything in coach, maybe even two AC's at the same time.
I have a different version of the adepter I linked to above and have had limited success with it. It was around $75 and is not something I would buy again.
The adapter evolvingpowercat references is a common marine adapter, at least what I think he is referring too.
Many older boats that needed more then a single 30 amp supply just had an additional 30 amp supply to a "L1-L2 or A-B" breaker panel.
Our boat is built this way and as such we carried two 30 amp twist-lock power cords and a 50 amp "Y" with two 30 amp females.
The adapter is needed because most marinas have all been upgraded to full service pedestals. Typically single 100, 50, 30, 20 amp service.
There is marine adapter that matches what evolvingpowercat referenced above. A two 30 amp male "Y" into a single 50 amp. These are made for boats that have a 50 amp shore power and are plugging in to a twin 30 amp shore pedestal.
They are special and are referred to as "Smart 50's". They are typically built with sealed relays that allow you to plug in one of the male 30's and not have the 2nd male 30 hot. Once both male 30's are plugged into proper receptacles power is allowed through to the 50 amp end. These are expensive, in the 400-600 dollar range.
Homemade versions of these adapters are deadly.
On edit, this is the adapter I carry.
http://www.progressiveindustries.net/cheater_box.htm
