I'm often amazed at what passengers (are allowed to) take on as carry-on baggage. Personally I hate travelling with a lot of carry on - it's just something else to have to lug around until boarding and it's something else to drop, lose, have break etc. There is also something infuriating as you sit in your seat waiting for the doors to close to have some imbecile bash your head as they walk past with their oversized suit carrier / mainframe computer / Wendy House / shoe collection. Then the joy of watching such people struggle to lift their valuable wares into the already stuffed overhead locker, packs of sherpas appearing to try to lift and wedge the baggage into place before slamming the locker shut and hoping the contents won't fall down breaking the necks and skulls of the row of people below.
Most places I go to (fortunately in Asia) checked baggage is usually returned quickly; I can sort of understand passengers in UK or USA where from my experience the return of checked baggage is dependant upon human sacrifice and may or may not happen if the carousel gods are displeased.
To encourage passengers to check their camper vans at check-in how about offering a financial incentive. If the bag is over a certain size the passenger has to pay a sum approx US$40 or local currency equivalent. The total sum could be split amongst the other passengers who are not selfish gits.