How can I cook more efficiently in a campervan using a single pot over a gas-powered flame?
I've done a lot a single ring camping cooking. My camper van has a bit more kitchen than yours but I still take a similar approach when staying in it and going to work. Here's a typical example that worked for me. 


I cooked a curry at home and put a portion in the fridge (freezing is also an option).
When camping I boiled water and put it in a vacuum flask. 
Then I heated the curry in my larger of two pans, with a lid on but stirring frequently. Heating gently meant it didn't stick. 
Cooking the rice (using the recently boiled water, and an excess of it rather than measuring) took place in the smaller pan, using the larger as a lid to keep the sauce hot. 
Then I drained the rice water into a mug or similar, added the sauce to the rice, and poured the cooking water into the sauce pan with some washing up liquid so it could soak while I ate and would be easy to wash up. (I used mess things to cook in and eat from when in the tent, it's easier with a few more containers). 


The same thing works with pasta. Noodles (ramen) cook much quicker than spaghetti, saving fuel, and make a perfectly acceptable substitute. The principle of keeping one thing hot over the top of the other is key, as is a vacuum flask. Washing up in starchy water doesn't work very well, but soaking and prewashing do, allowing you to wash up in a minimum of water. 

If you don't want to rely on taking home made ready meals there are plenty of books and blogs devoted to one-ring recipes (recipe requests are off topic here anyway). 

Pasta, rice and noodles all work quite well, needing no preparation. Frying can be a pain if you're cooking in the van - it makes things greasy and the smell lingers, but in dry weather it works outside (if you can take the stove out). A canned (jar/packet etc. ) sauce can be used with something like tinned tuna if you don't have fridge space. Defrosting in a cool bag can take 24 hours or more and it uses a lot of gas to defrost on the stove.