sounds like an infrequent loss of 'ground'... maybe due to corrosion, or just a plain 'loose' connection.
When the wires are in 'use' by the 12v system, they create some heat, which can create interesting situations(loss of power/connection) when they 'expand' or 'contract' during this time. Corrosion can probably creates some similar scenarios.
I could only suggest to check every possible connection upstream from the batteries. Yes, a relay or solenoid could be going bad, or 'unlatching'. Another idea is to check the wires behind the 'Use/Store' switch, itself, if you have one.
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the Turners...
two Campers, two Electric cars
former diesel pusher traveler
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