12v and Gas V's Petrol Generator

I've been advised to post in this section so apologies for double post.

We got our first caravan about 7 or 8 months ago so are kind of new to it all.
We have never gone without EHU so far but we are hoping to spend a night at Knockhill to see the BTCC this August. There is no EHU available and I've been toying with the idea of buying a 2.2kw generator but at around £500-£600+ they are a bit expensive for my wallet. Alternatively I'd need to get a 12v 19" TV at around £200-£300.
The essentials; fridge, hot water, lighting, heater (August can be very cold at Knockhill) etc, can all run on gas or 12v for a night no probs but I'm thinking longer term. I quite like the idea of not being tethered to an EHU but what if I'm away for 3 or 4 nights on a regular basis?  How much petrol would a generator use running the fridge, hot water, heater etc compared to gas, without trying to be TOO conservative?