Let me see if I understand this right.
Economy switch ON they won't take the load when the A/C starts (this is common)
Economy switch OFF they run the A/C just fine, but when the A/C cuts out the generators fault and shut down.
Suggestion one: SET A/C blower to HIGH,, this may fix it in fact
Suggestion 2. Plug in a 100 watt light bulb (or 2) somewhere inside the trailer. Put a "Ballast" load on the thing.
Problem (Suspected) when the A/C cuts off there is what is called an inductive SPIKE, the generator is reacting to that spike,, Keeping a load on it (especially a light bulb or other non-inductive load) will help..