My truck has two start batteries in it and I still plan to use another battery for the trailer. I'm thinking it gets down to where you plan on camping. All it takes is one night of oops I left everything on to drain the start battery. If you're in the middle of BFE, how long will it be before you can get a jump? I camped in the back of my truck using the start batteries for lights and the laptop and I couldn't stop worrying about draining the batteries. I think a lawn mower or "jump start" battery would be good just for the peace of mind at a minimal increase in weight.
We typically drive the car every day when we camp. The car battery should be fully charged each evening. The worst-case senario, I can envision is falling asleep with the vent fan running on some dreadfully hot night. At a rate of 3 amps over 8 hours, we're looking at a 24 amp hour drain. I don't know if that's enough to prevent the car from starting. Regardless, I'll get one of those emergency starting batteries just in case.
Ms. Swenson insists on the comforts of modern plumbing. Getting stranded in the backcountry is not an issue for us. We camp in established campgrounds. Usually state parks.
I'll put the connection for the external power cable at the front of the tear. That way, I can easily connect the trailer's electrical system to a tounge mounted deep cycle battery at a later date.