Generator clicks once instead of starting

From your description I am going to guess that this generator is in a motor home. If this really is the case then that means there is more than one place to crank and start the generator from. That means an outside location at the generator. Normally the switch is a rocker style switch. Push one way for on and the other way for off. The exterior switches get gummed up with dirt and grit and any other junk floating around, especially when on the road. The gummed up switch may not return to the neutral position when it is used and can cause a couple of different aggravating conditions like even allowing the unit to start from another location but then causing it to die when the switch is let up after starting. After you have cleaned up all connections and battery terminals take some electrical cleaner and spray the rocker switch located at the generator and work it back and forth. The interior switch(s) might need a cleanup to.