I've considered voltage drop over cable length only in as far as I'm aware that it occurs. I haven't tested the level of voltage drop.
I currently use 12v circuits in my camper van and haven't experienced any significant drop over cables about 3m in length. LED-based lighting and a 12v fridge operate as expected and the voltage at the point the power reaches these devices is very close to the voltage over the battery supplying the current.
I previously used 12v circuits in a narrowboat I used to own and didn't experience any drop over cables about 20m in length. This used halogen-based lighting and again the voltage at a lamp was very close to the voltage over the battery.
Just to be sure: You are aware that you have to measure the voltage drop under load? Even with rather thick wires, 20m should give you some noticable voltage drop with any non-minimal load.
I currently use 12v circuits in my camper van and haven't experienced any significant drop over cables about 3m in length. LED-based lighting and a 12v fridge operate as expected and the voltage at the point the power reaches these devices is very close to the voltage over the battery supplying the current.
I previously used 12v circuits in a narrowboat I used to own and didn't experience any drop over cables about 20m in length. This used halogen-based lighting and again the voltage at a lamp was very close to the voltage over the battery.