If Robg is only interested in a petrol model, then he will have to accept that especially when towing, he will experience significantly poorer MPG, than a comparable diesel powered vehicle.
This is also compounded these days, by the weight saving car makers are doing, to improve fuel consumption, lighter cars need less power to move then, thus reducing fuel needed. Unfortunately whilst this means we may see some cars with great economy, they have much less spare capacity to pull a caravan.
But having pointed that out, I can remember how back in the 1960's cars with much less power were towing caravans, and most of them were petrol, so it isn't a total lost cause. I suspect we have become used to having more than enough power, which has reset our expectations to a higher norm.
If Robg can be prepared to wait a year or two, there will be some PHEV cars coming onto the second hand market in his price range. Some have a modest towing capacity, but importantly, they seem to be petrol ice, but with the added benefit of the battery which for normal driving can make a big improvement to the economics. And the electric boost can provide useful extra torque needed to get a caravan moving.