First off - -
I hate to throw cold water on your plans, but I really believe you would very quickly find that four people and all their "stuff" will make that 18 feet feel more like 8 feet. Throw in a few days of summer rainstorms (
or worse yet, the dreaded "H" word!) and that divorce becomes more of a possibility.
We recently traded a 26' motorhome for a 30' model because the two of us, seniors with two little dogs, were just too crowded in it even for relatively short stays - extended weekends and an occasional week long stay.
Camping on a beach is possible where it's allowed, but keep in mind that in most cases you will not have water or electric hookups, and the water tank and battery will not last long between having four people and hot weather.
Again, I hate being the bearer of bad news, but I sincerely believe your plans, as you've outlined, would prove unworkable.