We have stayed at a few haven sites and they are fine. We normally go for a mid price van (I think). Definitely not top end. The beds are made up on arrival. My dc struggled with the bedding this year. The duvets were too big for the bed and the covers were to short to fit the whole duvet in. My friend stayed at a different haven and had the same issue. If this is likely to cause your dc an issue, it might be worth taking them a sleeping bag (My autistic child had a massive issue with the bedding not fitting the bed properly).
The best thing we did this year, which we have never done before is to order a click and collect Tesco slot for when we got there. Dh left me in the van to unpack and went to collect the shopping. I don’t know why we haven’t done it before! (But be warned, the fridges generally aren’t massive, so don’t plan on storing too much cold stuff). We had three pans, a couple of Pyrex type pie dishes, a plastic colander, a baking tray and a measuring jug. Cheese grater (although I had taken my box grater, which I find easier than the flat ones), veg peeler, scissors, a couple of knives (but not very sharp ones). I can’t remember what else.
I had taken sharp knives, scissors, tea towels (more than you think you need) oven gloves, ice cube tray, dish cloths, anti bac spray/wipes, plastic tubs for fridge storage, sandwich bags, tin foil, couple of pint glasses and my gin glass, chopping boards, Bottle opener/corkscrew, reusable straws (my dc can not stand using paper ones when we have drinks out), insulated mugs so you can take a hot chocolate out with you to have on the beach/up the mountain, carrier bags (if you don’t want to have to buy more), dirty washing bags, bin bags, kitchen roll, tissues, toilet rolls (we had two toilets, but only one toilet roll, washing up liquids, scourers. I took bleach to clean the toilet with.
A bath mat. Pegs. We had a Clothes drying Rack to hang out of the window, but things would have blown off if I hadn’t pegged them down. Hand towels. Coat hangers (no drawers and hardly any hangers, so we had to leave pretty much everything in the suitacases, Which wasn’t ideal.
I never thought to take the Hoover, we didn’t have one in the van and I wish I had been able to clean the floors as they got a bit grim halfway through the week.
We had no storage in our bathroom, I was glad I had put all the bathroom stuff in a plastic crate that I could leave in the hallway just out side the bathroom door.
I appreciate a lot of my list is unessential, but it Is stuff that would have made my life harder if I didn’t have.