Daytime parking is not an issue, huge car parks and you can either park with cars or the coach park. Ask one of the staff as you drive in.
We were lucky when we went a few years ago, they allowed motorhomes to stay overnight in the staff car park so we arrived late Friday night and parked up. Moved to the main car park for the visit and at the end of the day drove back down to the staff car park. No facilities but we didn't need anything... A real pity they no longer allow it or have planned anything with those huge car parks sat empty all night
We go every year [grandchildren quite obsessive and wildly determined to go AGAIN when we stagger back exhausted] about the whole trip] Site at Dorney [Amerden ]is lovely but I think it is not open until April. Last time we went we stayed at Scotlands House Bracknell; that was also very near the dreaded Legoland.
Both sites are clean, differ in facilities- but good websites.
Parking at Legoland is fine- the site is huge and the earlier you get there the nearer to the entry you will be able to park.
We are mainly concerned about the nearness of overnight sites to Legoland as we try to get there as early as possible, queues start on roads before it opens; we try to get there at opening time after a drink in the van so the kids can get on rides before it is crammed. Both sites we have used are ok with us arriving late ish the night before the trip ; both quiet, friendly owners and kids sleep fine ready for the next day.
If you havent been before make sure you plan the day like a military exercise- take own food in a wheeled trolley [as the food is expensive and not healthy /veggie and G kids eat like horses] Take towels for the splash pools and if you intend to buy lego as a gift for an after trip present get it on line before you go as the shops on site are much more expensive than they need to be .
Just a further thought ... we often took a picnic lunch on the trips but last time the gate search staff reused to let us through with our [ would have been much needed!]chilled bottle of white; met with another dear friend who was devastated by loss of liquid refreshment . They do serve alcohol in the legoland arena but it was not up to our old lday salmon sammie standards.