I now use a RidgeMonkey fold-up 80w solar panel that charges the leisure batteries via a victory charge controller. It was happily pumping out 55w in full sun last Friday (250wh over about 6 hours).
https://ridgemonkey.co.uk/product/vault-c-smart-pd-80w-solar-panel
Charging via the cigarette lighter sounds like an expensive and potentially dangerous accident waiting to happen.
I’ve also tried a roof mounted panel but that failed after about 18 months and damaged the roof too (to my mind it wasn’t fitted properly- even though I paid to get it done ‘professionally’). I have a replacement panel kit from Solar Camper Solutions but I’ve decided not to fit it. If you want it, pm me with a sensible offer and it’s yours!