One thing you need to remember is that the Airstream aluminum finish will not reflect the sun as well as a white painted trailer. I had a 13.5K unit on my 27' Safari which in my opnion was undersized as you approached temperatures of 90+ degrees in full sun.
At one trip where we were approaching 100 degrees in full sun, my unit barely kept the interior at 85 degrees. My neighbor in a similar sized SOB with the same sized unit was in the upper 70's. My dealer, Airstream at Jackson Center, and the manufacturer told me that the unit was operating within specs. From my own testing I agreed with them. I believe the unit was just undersized and unable to deal with the heat gain occuring in a full sun extremely hot midwest summer.
When I ordered my Classic, I upgraded to the 15K unit. I'm glad I did.
My feeling is a 13.5K unit is probably the best size for your 26' unit. Obviously if you are going to be in areas of extremely high heat or full sun camping, you might consider the 15K.
'04 Classic 30' S.O.,'03 GMC Savana 2500,'14 Honda CTX 700