Sounds like the exhaust from the generator is getting in the trailer. Are windows or vents open ? If so the wind may blow the exhaust in.
You could try getting a piece of conduit or flexible conduit that will fit over the exhaust pipe to extend the exhaust away from the 5th wheel to see if the alarm still goes off. I used a 10 foot long piece of flexible conduit to run the exhaust away from our 5th wheel so we didn't smell it when sitting under the patio awning.
If routing the exhaust didn't work you might want to check the generator box/storage compartment for any possible leaks into the 5th wheel interior.
