Follow up on prior posts.
1.If you do attach a backup battery ala Vee Jay don't try to use it to start the engine by itself ... that will blow out the master 30 amp fuse for the fuse panel. (Inquiring minds want to know this stuff.) This "spare" battery is basically our night run battery to reserve the house batteries.
2. The Green Motor Fuse holder talked about in the original note that started this thread has a spare fuse inside the fuse holder. I rethought out the wait of ten years to check this fuse next ... the existing motor fuse and spare looked new after 25 years.