SIMs instead of blow up mattresses. Warmer, comfier and avoid you bouncing around every time your partner moves.
Duvets instead of sleeping bags. Much cosier and more comfortable. Put one under you on top of the Sim and the other one over.
Get EHU and take a little fan heater. Keeps the tent toasty.
We have a large tent and use the side porch as our tent toilet area - we have a fold-down camping loo, can't remember the brand but basically you sit on a toilet seat over a box, which you line with a heavy-duty bag with water-absorbing crystals in (it comes with a supply). We don't use it for number 2s(!) but it's invaluable for middle of the night wees. Nothing worse than the 3am trek to the site toilets!
Blow up sofa makes life comfier in the evenings.
Vacuum pack your clothes, duvets, blankets etc in those special bags to save loads of space in the car.
The big blue IKEA bags are brill tent storage. I use one for each of our clothes and have one on the go for washing.
Camping towels seem like a gimmick but are actually brilliant. They're microfibre, dry you really quickly and are dry themselves in less than an hour. Saves having wet towels draped everywhere in the tent.
Crocs are better than wellies on campsites.
And we have a trailer to put it all in!