Just to be sure, though I suspect it is true: you did wait a minute or two after starting the generator for the power to appear, right? There is a time delay (typically around 30 seconds or so) on a transfer switch before it switches to generator power so as to let the generator get up to its proper running speed before applying a load to it.
If you happen to have a gizmo like a PI EMS wired after the transfer switch that also has a time delay before applying power, those two time delays will happen in succession, so it could be a few minutes before the coach has power in that case.