Old Orchard Beach Maine caters to families with children. There are several campgrounds there and I think at least a couple of them can accomodate you. Bayley's Camp Resort is the most well known and gets good reviews form most people, but I've never stayed there myself.
http://www.oldorchardbeachmaine.com/
As for "lobsta", you would be hard pressed to find a Maine seacoast town that did not boast several lobster spots, anywhere from a shanty to a restaurant with white table cloths.
Then there is the Maine Lobster Festival every August in Rockland, ME. See http://www.mainelobsterfestival.com/
There are several campgrounds in the area - one of the nicest ones is the Camden Hills Rv Resort in Rockport, a few miles up the road. Pricey - but most New England campgorunds are. Camden Hills has sites plenty big enough for your 43 footer and the local restaurants in Rockport and Camden are excellent. Camden, ME is a classical Maine seacost village and the port offer schooner trips, lobster fishing "eco-trips", etc.
We workamped at Camden Hills for two years and love both the area and the RV park. See http://www.camdenhillsrv.com/