When are you thinking of coming this way? Next spring, or summer?
Sampson State Park is on the east shore of Seneca Lake, we've been there many times. There are no FHU sites, but they are electric. A potable water source is there, to fill up your tank. I'm sure there are other private campgrounds around, also, but this one is right on the lake. you can check it out here:
https://newyorkstateparks.reserveame...archResults.do
Cayuga Lake State Park's nearer to the town of Brutus, also, but fewer electric sites available.