There are a number of "ifs and buts". Presumably, you want an electric hob/ring so that you can cook on the mains when on an EHU.
Is the current hob an integral part of a free standing hob/oven combination or is it mounted in the worktop as a separate unit? Is it your intention to fit a gas/electric hob into the work top if it is the latter setup? If it is the latter, will the replacement be a simple "shoehorn in" exercise? If not, it may entail a replacement worktop and a cut-out for the hob would have to be made.
Whichever situation obtains, it would be an expensive exercise.
Have you considered another solution? Purchase a separate, free standing, plug it into a socket hot plate? There are a few on the market. The one which springs to mind is marketed by Kampa UK Ltd, Witham, CM8 3EU (kampa.co.uk). It is rated at 1000watts, has a temperature/thermostat control and comes complete with 13A plug ready to plug in and cook. In our local caravan accessory shop they are priced at £16.95. Kampa also market a double version but I have not got any details on that product. No doubt the spec. and details are on the Kampa website.
This may not be exactly what you are looking for but offers an inexpensive solution without all the hassle of purchasing a 3 gas burner/1 electric hob and perhaps have to replace/modify the worktop. It also has the advantage that you could use it in the awning if you have, or can organise, a suitable 13A socket to plug into.
By the way, I have nothing to do with Kampa UK Ltd. I just happened to see the unit and thought that it was a good idea and a simple way to be able to use the EHU for cooking. At £17, it is not going to break the bank. So I bought one. I've used it in the kitchen at home as an experiment and it works very well.