[Don L]

Tests: Start battery at 12.6 or 12.8 volts between posts. When cranking, voltage drops to low 10's, possibly high 9s. Combining house and starter batteries using the Blue Sea switch does not help. Having fully charged batteries does not help.

Where did you measure that low 10s/9s at? If it was at the battery terminal you have a bad battery. If it was at the starter you have a wiring problem most likely a terminal. If it was at the starter solenoid you have a bad start switch, wiring problem, or a connection corroded.

If your start circuit takes power from the battery to the start switch and then to the solenoid any little corrosion in that circuit will cause a start problem.

 

[Don L]

a relay only adds a couple of feet of wire

but if the problem is a bad connection from the switch to the solenoid in time the relay even wouldn't get power (still worth having the relay for times when starter battery voltage is at it's lower limit)