Lets talk REAL world experience. I have been working on motorhomes as a professional for 35 years. In that time, I have seen Motorhomes before they had auto transfer switches that had manual breaker kill transfer and the old standard 30 AND 50 amp plug in receptacles in the power cord compartment that you plugged in the shore power cord to get Gen power to the RV. Then, they developed Auto transfer switches(30 and 50 amp models), I have started Gensets with Shore power connected. I have connected Shore Power when the Genset is running. I have done this at least thousands of times over the 30 years auto transfer boxes have been available. At no time did anything burn up or a catastrophic event happened. IS IT POSSIBLE? Yes, but extremely remote. As to what an Owners manual states. That it a CYA disclaimer. That just attempts to remove any possible responsibility and to deny a claim if something happens to the 120 side of the RV or appliances. There is NO way if something happens that they could pinpoint the problem to attempting to run the Genst while plugged into Shore Power or Vice/Versa. Doug