Originally Posted by mowmow
You're going to need to know how your vehicle (and every AC appliance in it) is wired in order to answer with certainty. You're also going to need to know how the pedestal is wired.
It's possible that the pedestal is powered with a single 120V, 30A feed that is fused at the source at 30A or 40A. Therefore, the same 120V leg is the source for both the 15A and 30A receptacles. In this case, trying to fully load both receptacles will blow the breaker at the source (30A+15A is greater than the feed breaker) and cause the campgrounds owner to wonder what's going on (why didn't the campsite breaker trip instead of his 40A breaker?).
Maybe the feed is 120V single leg, fused at 50A and the feed wire gauge supports this amperage. In this case, you will be able to get the 45A, but this again is just a single leg from the source. Any appliance that runs off of 240V will not work, and I don't like the idea of running the same leg/phase to both legs on that appliance - it obviously won't work, but even if it's not in use..
If the feed uses both legs of the 240V mains (one for the 30A, and one for the 15A), then you have a decent chance of things working fine on any RV as long as you don't exceed the current limit on the 15A leg. I still don't like this much, as an overload could only pop the 15A breaker. So a 240V appliance in the RV can feed voltage from the live leg to the dead one. This can result in all kinds of strange voltages (less than 120V) which could possibly damage certain equipment in this brown-out state. This is why 240V breakers have both breaker legs mechanically tied together - one side failing trips the whole thing.
Why is GFCI not supported? 120V GFCI's constantly (millisecond by millisecond) compare the current flowing out_of_the_HOT_wire to the current returning into_the_NEUTRAL_wire, and vice-versa. If those currents don't match within 10mA (0.01A) or so (as in someone holding the HOT wire while grounded), the GFCI breaker will trip. Using this adapter can allow current to go directly from one AC leg to the other (through a 240V load), bypassing the GFCI's neutral path altogether, which would result in several AMPS of imbalance on the GFCI, thus it trips immediately, leaving you with only the the leg that doesn't have a GFCI as well as the potential brown-out situation that I mentioned above. The adapter (depending on how it's actually wired) could also feed current from the 30A leg into the GFCI's neutral wire, which would cause a 15A GFCI trip from a 120V appliance running normally on the 30A leg.
Regarding 45A being enough POWER to run multiple A/C units, etc: Amps is not power, WATTS is (are). The reason that larger RV's adopt 50A service over 30A is not because it increases the power by a factor of 50/30 (1.6 times), but because it increases by [50Ax240V (12,000W) divided by 30Ax120V (3,600W)], or 3.33 times. So, the 45A example can power 5.4KW [(15A + 30A) x 120V] worth of equipment, versus the 12KW from a proper 50A pedestal.
Why do they use 240V appliances then, when they can cause these issues? Lower current. For a given power requirement, higher voltage means lower current (power = voltage times current). Lower current means smaller wire size, smaller connector pins, and less heat and power loss in the power delivery system.
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