Hello S. Dronsfield, For the most part portable generators are very inefficient at converting expensive petrol to useable electricity, despite the claims for super silent they do make noise, and the fumes are just as deadly as a car exhaust. In general there are far more efficient and quieter alternatives already installed in your caravan, Use gas for the fridge, water and space heating. The only general items that may need 230Vac could be a telly, or computer, and there are 12V to 230 inverters available that will run those items for you.
Having said all that, there are times when a generator is the most effective answer, so it is quite easy to wok out what you can use. All electrical appliances will carry a data plate which will state the power consumption of the product in Watts. Simply add together the items you want to use, and check that they do not exceed 650W. You are not likely to damage the "board, if you over load the generator, but most generators have an over current trip switch that disconnects the 230 v circuit.