Mine are on a 20 foot 10 guage tether. I have measured as much as 22 volts in full sun with this arrangement. (4 15 watt panels, total 60 w.) 20 feet should be more than enough to find a spot in the sun. Unless you are in a parking garage...
loss per foot.. don't know exactly, but keep in mind, your controller is going to limit the input to the battery at 14.5 volts. I'd say anything OVER that going into the controller is for good measure, but you will do OK in the 12 to 15 volt range. (Depending on current draw and how fast you deplete the battery.)
Oddly enough, using my "Old" 15 watts panel, I got didly in the shade, but in overcast, I still get over 12v. Have not seen it drop below that at anytime when out from under trees etc even in a rainstorm.
I notice the same with my newer larger array. To the "eye" it seems to be the same brightness out, but the direct solar UV is the key.
Solar is NOT the end all to free power needs. It will work great for you as long as you know the limitations and learn how to conserve, (Not go without, but CONSERVE wisely) install less power hungry items and be prepared to cut back at times when the sun is not co-operative.
Things I did to conserve:
Installed LED lighting
. Soon to have cold Cathode like Mikes.
Have back ups for powered systems like hand pumps etc.
Swapped out my powered vented cat heater for an unpowered one.
Use invertors matched to the task (Like only having a 100 watt one for my comp instead of running the 400 watt one I need for the TV/DVD)
Fantastic Endless breeze fan as well as a roof vent fan. The roof vent one is a power sucker, the endless breeze runs for days on a single 20Ah Gel Cell.
Speaking of.. back up batteries for non essentials LIKE the comp.
Car is wired to charge the house battery. In a pinch, I can always run the car connected for a bit to help things along.
I am like you, I don't want them on the roof for various reasons, but one of the biggest is what happens when parked in the shade.