You're doing it all wrong. 5cm (at least) thick self inflating double mattress of almost any brand. There are some 7.5cm ones around.
Two square sleeping bags, one with a left hand zip, one with a right hand. You can then zip the two together. Just because you're camping, you don't have to pretend you're in an old film where husband and wife have two single beds…
Decathlon do S20 and S15 sleeping bags cheap, so you could get a mismatched pair if she's nesh.
The whole point of car camping to me is that you can make it comfy, take a 4 man tent for 2 people, have thick airbed, picnic blanket, 2 ring cooker etc. so that it's viable as an actual comfy holiday.
PS, don't buy camping chairs. Get Ikea's folding exam style chairs. And Decathlon's folding tables.