The first port of call is the site warden,but if they are not around you could try an old trialing car trick. Let your tyres on the drive wheels down to about 15 PSI ( half flat)this puts a lot more of the face of the tyre in contact with the ground, then without spinning the wheels feather the clutch and you'll be suprised how much extra grip you get. Remember one thing though the first time the drive wheel spins on grass, all the tread of the tyre is filled with mud and so you might as well be on racing slick's. I have seen guy's reving like mad and wheels spinning all over the place, all they are doing is ploughing the pitch and going the right way to be chucked of the site!! There's no shame in getting stuck and asking for help, a quick tap on the door of most caravans brings out a willing team of helpers and all for the price of a cuppa. I've been caught out a few times towing our twenty foot twin axle van with a front wheel drive car, but the advice above has always worked for us. Have fun.