My son has recently purchased a toyota corolla hybrid, the car is to small for my liking but thats not to say im not impressed with it, with the economy at around 30 km a litre it cirtainly goes a long way on a tank of fuel, the rav4 would suit me with very similar economy.

Im wondering if anyone here has had or got a hybrid and can give me an idea on the economy once a trailer is used for kayak transport or on the roof?

I own a hybrid but only a Prius so not used for towing. The range of hybrids available in AU is very limited at the moment but should expand in the next year or two, especially as BEV's also come into the market. A hybrid is effectively just a more efficient ICE (internal combustion engine vehicle) and much of the time on extended highway driving you will mostly be running on the petrol engine. The battery electric component has very limited range in a hybrid and mainly comes into play in stop start driving and for the first part of acceleration.
Btw my next vehicle purchase will be a BEV, most likely later this year and not for yak use.