I have run into this issue with cars, boats, and aircraft. Even with a fresh battery, and clean battery terminals, the voltage during cranking can drop to less than 8V on a 12V system when cranking a large engine.
I have used the following trick: a 2Ah 12V SLA isolated with a Schottky rectifier between the starting battery and the load that must be continuously powered > 12V. While the engine is running, the alternator gets the terminal voltage of the starting battery up to ~14.2V. The forward drop of the Schottky means that the SLA is "floated" at ~13.7V while the engine is running, which is about right for keeping it charged. I used a small relay to automatically disconnect the load (GPS, engine computer) when the master/key switch is finally turned off. The next time the key/master switch is turned on, the (GPS, engine computer is powered from the SLA/Schottky/Starting Battery. During cranking, the starting battery can be pulled down to 8V, while the SLA keeps the loads above 12V...