Yes I know, I'm a prime example of a bull in a china shop - not even completed one camping trip yet and I've already gone and broken one of the knobs on the gas locker door. I thought it was unlocked, rotated it perhaps too aggressively and ended up with it in my hands.
Now for the question. this knob uses the same key for opening it as for all the other door/locker knobs so how does one go about replacing the knob but still be able to use the original key? Are they only available from Hymer I wonder?

that's a good question?
there may be a u shaped clip that holds the lock to the handle, that will slide out
i remember fords car locks (slightly of subject sorry) you could buy the locks separately and had to assemble the parts in the correct order for the key to work, anyhow that lock was held with a u shaped retainer

sorry was about to say have you looked on ebay at the hymeruk seller who may sell a knob , then thought perhaps your main dealer may be able to supply under warrenty

No, there's no U-shaped clip or anything like that Pop. Something physically broke off inside the outer part (the images will make this clear) and essentially the outer part just rotates freely. The outer part (the key part) is held on by two screws which do not screw into the inner part - the part inside the locker itself. This part is still in place and I don't know whether it's actually glued on or is just sticking there on the inner door face.
Either way, the outer part - the bits that broken - no longer connects with the inner part and turning the key does nothing at all. Obviously I'd like to obtain a replacement inner part in order to be able to continue the original key but they doesn't appear to be a way of removing the actual lock barrel from the handle itself. Um.
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Your dealer should be able to get a complete matching set including lock to replace the broken one. The lock barrel should be able to be removed with a special key but only the factory appears to have the key. So best option is new barrel you will need the key number to order it

Sorry, a bit remiss of me not to get back on this subject with an update. Anyway, my dealer made such a botched effort of extracting the lock barrel that I rejected his offerings and insisted that he order a replacement part from the factory. Registration document needed to be presented as proof of ownership along with key number and a replacement was delivered within the week. Can't remember the exact cost now and I'm too idle to dig out the paperwork this late in the evening but it was much less than I had been expecting.