Our Top 5 already been covered above I think. I’ll ignore your crockery and glasses, kettle and bottle openers which are the no brainers. But I’d say at the start, use as much from your kitchen until you find out what you actually use.
1. Levelling ramps (2 sets of the yellow Fiamma ones - often needed to stack ramps to get level, and smaller ramps easier to pack).
2. Thule hold down straps for the wind out awning. Used them often, saves an expensive repair following gust of wind.
3. Good quality duvets, 2x singles, in case we each use a different bed (top, bottom), but fit fine when sleeping in the same bed. We went for natural wool fillings, very warm, the filling is hypo allergenic and self cleaning, and it’s planet friendly.
4. Topper for bed(s), or self inflating mattress. We’ve tried: VW mattresses on their own for quite a while (sleep wasn’t too good/ comfortable); Outdoor Revolution Rock n Roll self inflating mattress (much better sleep, okay downstairs, but a pain to deflate upstairs - even with a Outdoor Rev mini max pump + Cali shop adaptor), but also great to use in the side awning; Duvalay travel toppers (2x singles 58cm so they are easier to pack away) - not yet tried - but hopefull they’ll be as good as the OR mattress, easier to use, and pack to a similar dimension.
5. As you’re winter camping, internal screen and side window covers, Brandrup Isolite type. Very good insulation, don’t get wet (on the inside, and prevent condensation - mostly), good blackout properties, pack flat in the pop top.
To be honest, No.5 was difficult - we’ve got a few more initial must haves!
A windbreak is fantastic, allowing you to sit out / cook more. We bought an Outwell poled type, it packs very small, but is 1.2m x 5m long when erected. It will change your life on site.
Quick assembled awning tarps - similar to windbreaks - would change your life. Minutes to attach, pack away in your chairs in the tailgate. Various types on sale, we got Outwell Hillcrest and Fallcrest, about £200 so not cheap, but they also keep the rain off and expand your outside space.
Also as you’re winter camping, 3-piece rubber floor mats are essential, as may be a set of VW waterproof seat covers. Ours were given by our dealer, but I think each is about £100 - £120.
If you’re cooking, the first pot we‘d advise is a Ridge Monkey deep combination griddle and pan. We’ve cooked so much in this - basically anything you’d use a covered casserole dish for at home, and more. 2nd purchase would be a Ridge Monkey toastie pan (middle sized one).
If you’re planning to stay on sites with EHU then a small fan heater and thermostat. Saves your diesel (although the diesel heater is very good). We have a Kampa Diddy plus separate stat.
Cheap storage / organiser trays for the kitchen cupboards - Dunelm grey plastic type - combinations fit the cupboards.
AND don’t forget…. Steel hammer (lump hammer) for pegs - even used for side awning pegs), and peg puller; hook up cable; short water hose (blue hygienic) about 5 to 7m plus hozelock fittings; and - sunglasses for the optimistic winter camper !!