Getting into shade will do more for you than another 8 to 12K of cooling power. Difference between heat load in sun and heat load in shade may be as much as 30K to 100K BTUs, and it is really difficult to add that much cooling power.
I would be inclined to at least double the cooling with a second full size RV air conditioner, rather than fiddling with a litte one, and you do have to find a way to provide power. My experience though, even a second 15K of A/C does not help that much when caught out in full sunlight on hot days.
Cools down nicely after dark, though.