I use an adapter from the 30A plug to a 20A regular plug and plug it into an extension cord and then into an outlet in the garage. That will (in my case) charge the batteries from shore power, allow me to run the A/C, ceiling fan, etc., including the ridge. The 20A circuit is just adequate for the minimum power requirements - no TV (or TV and no A/C), no microwave - just charging the batteries and keeping the fridge cold. If it runs the A/C, all the better, but it may take some power management to get things that you want, i.e., turn off the A/C so you can use the microwave and vice versa. If your rig doesn't have the adapter, they can be found at the camping parts places, Home Depot, Walmart, etc. I know of no rig that doesn't charge the batteries while on shore power.
That is the scenario when parked in the street getting ready to go. When parked in the yard I have a 30A circuit that I trenched in to the parking area and I leave it plugged in 24/7, with everything off - it keeps the batteries charged and a fan running to keep the humidity at bay. When I go out to the trailer to get it ready or whatever I turn on the A/C and whatever else for comfort while I'm out there.
2002 Holiday Rambler 30RKD
2000 F250 diesel extended cab short bed
2014 Demco Autoslide 18K