I am in process of doing just this same thing on some land we just purchased All I am doing is running a 4in line to side of slab & turning up , then putting on cap to seal while rv gone.
I all ready have a adjustable 3in pipe with rv connection on one end & a 3/4 rubber reducer coupling We use at our rented site at the lake.
You can just use your flex hose to hook up but long term will deteriorate quickly in the sun.i wouldn't worry about amount dumped at one time if you just using your land as occasional get away place as we will do