While my house was under construction, I desired to prewire for under-eave camera mounting too. I had the wiring contractor run combination wire (RJ59/power) to the eave and let it hang down. Then I asked the siding installers to mount a 2x6 board in the eave (with a hole drilled for the wire) just above the soffit location. I figured this would give me something to drill into with screws long enough to reach past the up-down pattern of the soffit.