Have you tried the likes of the Holiday Inn or similar hotels as sometimes they allow up to 15 days free parking if staying at their hotel as they do if flying out of many of the major airport.
Hi Why bother go by Evesway coach try there web site it will be dearer than car travel but less stressful,load your luggage on the coach next time you see it is in your cabin?
Depending on when you booked your holiday e.g before Sept last year P&O were offering free parking - assume you have booked since then.
We caught Evesway from Nutsford services and after being a bit sceptical about travelling on a bus it was great, no hassle and you turn up on a staggard arrival and the same with departing the ship.
Phone P&O and see if you can have free parking.