A tape measure is your friend here. Measure how much straight awning rail you have. Measure how much straight rail you have rear of the door. Assuming the door opens toward the middle of the van you don’t need much awning rail rear of the door, just the width of the pads that go flat against the van. Measure what sort of distance you have between doors and windows. For some, a porch awning dropping over a window is a deal breaker, others (like me) are less bothered but it is worth avoiding if you can. Measure the height from awning rail to ground. Armed with all these measures go to a decent awning retailer. I have had good experience of Renishaws in Clay Cross Derbyshire. Attwolls have lots of videos on you tube. I haven’t used them but they seem to have a good selection. Having found one that fits you can then search the second hand market to save some dosh.
Good luck. Just do the maths.
Mel