The car park for cars is quite good and i have seen lorries and caravans parked in it before but you have to go longways and it does get very busy,the lorry park is by the side of the garage and only has 3-4 spaces and cars usually fill this up as they fill up then cant get into the car park.
As already mentioned you would have to double back on yourself, you would get off the sliproad before you hit the m3.At least as you drive past the first time you would see how busy it is.