Two things:

1. Sketchup can export to DAE just fine, the only catch is, before you export, you have to right click on the part and select "Explode". I'm not sure what this does, but most of the time, doing it once isn't enough. Keep right clicking and selecting Explode until the Explode option is not available/greyed out. Once you think it is done, for good measure select all and try to Explode once more. Then you should be able to export to DAE and open it fine with Gazebo or Blender (I've actually never went straight from Sketchup to Gazebo, I've always had to do some editing in Blender, so I can only vouch that Blender will be able to open the DAE, then export with Blender and open with Gazebo, but straight from Sketchup to Gazebo might work.)

2. I can't tell exactly what is happening in the pictures, are you refering to the quadrotor being present in one pic and not the other? Sometimes the normal of a face gets swapped around, and most 3D viewers only draw faces that have a normal facing the camera. If that is what is happening, one way to fix this is in Sketchup, when you export (DAE), select the "Export Double sided faces" opption or something like that. Or in Blender you can flip individual faces which will result in a smaller file size because you won't have redundant faces that never get displayed.

I would try number 1 first because it seems more straight forward than all the other steps you were doing, something funky may have been happening in one of the conversions you were doing. If that doesn't fix your problem, mess with number 2 a little. Let me know here if you have anymore problems.