Concerning rough idle in caravans: while spark plugs can last for 3 years, the other tuneup components - distributor cap and rotor, do wear and only last about a year. Cracks or resistance and build up in the caps.
I also assume you don't have a bad spark plug wire, but that's also a possibility.
Also, the carb cleaner put into the gas only cleans the fuel injectors. The real area (as far as a bad idle is concerned) that should be cleaned is the throttle body where the idle "motor" is located. Remove the air intake hose to the throttle body and using a can of carb cleaner ($3.00) spray the stuff directly into the throttle body (past the butterfly valve) Also, into any holes you see on the inside bore. There's a small hole before the butterfly valve that is an air bypass for idle control. Carbon deposits will build up from the PCV valve and get all thru the inside areas. It will surprise you to see all the black soot that comes out.