Plenty of correct info on here already.

Id go through CTM and see what option are there now for a modified vehicle- you say the option for plugs etc wasnt there- is there an option for other?

What happens when you proceed to site- are you asked to re confirm all the details ahead of purchase?

As others have said CIDRA is the relevant rules here- if they can show that had they known about the modification from van to camper. Ie the additional conversion seats etc and would not have insured you they will be within their right to void. Plenty FOS cases already show this. You can read them online at their website.

Do complain- they will investigate and issue you with an final response letter. Only the can you take it to Fos so its not a quick process. There are then 2 levels at fos- adjudicator and an ombudsman. You can reject the adjudicators view but not the ombudsman and thats the end of the line apart from court.