[Piotrsko]

If you pump power backwards through your array it becomes a very inefficient infrared heater (which is how people melt the snow off their panels) which may or may not be significant to your panels. shouldn't matter forward the array just adds wattage to the system. Cigarette lighters may or may not be switched, mine are live always, but confirm the fuse size. Mine use 20 amp each.
Expect the system battery to add power to the starter battery so at least 8 gauge. I suggest fused and independent of the vehicle system.

How much fuel do you think you'll be saving? I calculated accessory load in the ranger to be <10hp for battery, coolant and hvac so maybe a quart of fuel an hour. If youre in the 50mpg class yeah it's significant, not so much down at 30mpg.