I had the same problem with a set of decals last month and found the sun-bleaching answer here on the forum, and yes, it does work like a charm! Only took about 10 days, and that is in winter in the Pacific Northwest (we got lucky and had a dry, sunny streak of weather).
The condensation might be a risk, so I did two things to help prevent that:
1. put the decal sheet into a zip-loc sandwich bag
2. added a little silica packet like comes in electronics packaging. I "dried" it out in the microwave first
I put mine on the dash of my truck, which I figured would get more sun (horizontal) than a house window (vertical). Had no vapor problems and the whites came out white. If there are other colors on the sheet you may want to cut them out first though, as red especially might fade. Good luck!
George