£700 for a tent – are you looking for staff to get it sorted for you as well? That is a serious amount of cash for the tent – considering your roofbars are being priced at £75 – does that include all the fixings to attach it to the roof?
You are looking for the convenience of a camper but not in a camper as you don’t use it as much. Given the alternative isn’t quite as convenient as a camper, are you sure you’ll still do this? If the current situation is as ‘easy’ as it can be, what makes you sure you will use the ‘solution’ more often when it is less convenient?
Roof box will need added to the car (and packed); tent will need to be packed then your clothing and gear for the few nights away. It sounds like there is more ‘faff’ without the camper van.
What kind of camper van is it?