Yup, hindsight being 20/20, you were in the second best place on the earth to break down with a dead battery. Walmarts have all the tools you'd need to replace a battery, and all the batteries you'd need to replace a battery. Even better an auto care center with people who would do the job for you.
Most of us wouldn't have lost more than an hour or two in that situation, and next time, you won't have to!
As far as using the camper battery to jump the truck battery, if you can physically access it, lift it, and carry it to the front of the truck, it will work in a pinch. When you're stuck on the side of the road with a dead battery, a battery is a battery. As long as it didn't get drained too.
Battery disconnects work both ways. The batteries are disconnected. "Automatic" disconnects are set up to disconnect with the key off, connect with the key on. That way the camper battery charges when the engine is running, and does not get drained if the truck battery goes bad, and does not drain the truck battery if you use too much power.
