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Jim, if it draws 16 amps, yes, you want a 20A or 25A fuse. And you choose the fuse to protect the wiring, not the device. Forty feet round trip at 20A would call for 6AWG wire, not 8G, so even without the bit of thin stuff you're getting excessive voltage drop.
And a 20A fuse would be just fine in terms of protecting the 6AWG wire run, too.
But all it would take is one corroded fitting, or one half inch of punked out wire in the 14g section, to kill the power anyhow.
Motors all suck more power when they start up, they look like a dead short initially, but it would be able to get past that and spin up if there wasn't a bigger problem. Take a voltmeter and measure the voltage right on the contacts to the macerator. I'd guess that even with the motor running, it should be within maybe 1/2 volt of the battery voltage, unless there's a problem (corrosion, wire, contacts, bum motor).
If you repeat that test with the motor hooked up to a bench source or right to the battery and the results are different, then it definitely is a wiring problem.