Powering Multiple loads from a single 12v battery
If it's only the motor starting current that's the problem, then it's possible to use a diode/capacitor (preferably a schottky diode for low drop) in the feed to more sensitive components to 'ride out' the brief dip. Calculate the size of the capacitor to power your sensitive loads for the duration of the dip.

If however the motor run current is also too large, then you will need to precede your sensitive components with a boost converter that will work down to the minimum voltage delivered.

It may be as cheap to use a diode + second small battery (think of the battery as a very large capacitor indeed) to power your sensitive stuff while the main battery is dealing with the motor.