When I had a Bluebird and drove coaches for FEMA (Coachman) I was always told to never start Gen. when plugged in to shore power.Why and is it the same with my Barth? Thank you.

There should be a cutover relay that powers the coach from one or the other; this also disconnects shore power when the genset is under load, preventing back feed.

Because the Automatic Transfer Switch is likely to fail eventually, manually preventing two current inputs is prudent. Some of the Automatic Transfer Switches, such as Parallax ATS 100, were available with a thermal delay module as an added safety measure. My current coach is so complex that is has TWO ATS boxes, not just one.

An idiosyncrasy of some, prior to the modern ATS 503, was that the relay points would bounce open if you were running your gen set while going down the road. No damage, just no more power to your roof AC or whatever.

My Regal has no automatic relay. There is a manual switch to choose either shore, genset, or off (neither). There is no way to get both sources at once. If you have this type of switch, you will have to manually switch over every time, but there is no way to harm either source.