Originally Posted by Grumpy
Unless the 20 amp is a ground fault receptacle, it will trip the ground fault.
I could be wrong but I don't think it will trip the GFI outlet. GFI outlets work by detecting current draw that does not return through the neutral line (E.G. if someone is grounded and touches the hot line). In the case of the Y cords the neutral lines are usually connected together as are the ground lines. The two hot lines are kept separate in the wiring and in the panel inside the coach. In theory, if the Y cord had only one neutral line connected - say from the 30 AMP side and left the 20 AMP neutral open with no connection then you could (most likely would) trip the GFI outlet on the 20 AMP side however there would be a larger problem with such an arrangement that is current draw. In such an arrangement you potentially could exceed the current draw on the neutral side because you should not be attempting to pull 50 AMPs from 30 AMP rated wiring. It isn't just the hot line that needs to be correctly sized it is also the neutral or return line. Anyway, having 50 AMPs in the coach providing the ability to run the coach on any of the following combinations would be very nice:
Two 20 AMP circuits
Two 15 AMP circuits
One 15 AMP circuit and one 30 AMP circuit
One 20 amp circuit and one 30 AMP circuit
One 50 AMP 240V circuit
The above combinations give the OP maximum flexibility over the standard 30 AMP single phase setup in most RVs. Obviously, even with the two legs available if it does not add up to the full 50 AMP needed then you can't use everything in the coach however you can still use more than you could if you only had a single phase 30 AMP setup.