Do you still have the PDF for replacing the evaporator? If you can email it to me that would be awesome. ThanksYou may be right that there is a leak at the evaporator, but I sure would NOT recommend that you remove it yet. Before you commit to that, you need to buy or rent a sniffer - I have one of these and they work great.
Electronic Freon and Halogen Leak Detector
You can download the manual and read up on the tool: ... http://manuals.harborfreight.com/manuals/92000-92999/92514.pdf
A total leak down in 36 hours is a major leak, and this thing will definitely pinpoint it. It has seven sensitivity settings, so you can adjust for ambient conditions and the leak rate.
If you have to, you can drill a small hole in the evaporator housing close to the coil and stick the probe inside. That way you will 100% know that there is a problem. Can you imagine how you would feel if you spent many hours sweating and swearing, and found that the coil was OK?
Since your filter was missing, you need to flush the coil with Frigi-Clean. It's a great product, but shop around since prices are wildly variable. Of course, if you have to replace it, then you won't have to buy the cleaner. The cleaner is an expanding foam that you inject into that little hole you drilled....
BTW, did you check for dye dripping out from the evaporator drain tube?
I can send you the blower motor and evaporator removal pages (pdf format) from a 2007 service manual if you like - I don't know if there are any differences, but I suspect there are not.
Spokane WA USA