Tell me why this wouldn’t work

Hooking up a 1200 watt inverter to the motor battery(only turn on while engine is running)to power the onboard charger for trolling motor batteries.

I’m looking for a cheaper charging solution than a generator.

How long are you going to be running the big motor?
I doubt that you will see any benefit running the limited time that most run the big motor. Unless you are at Rayburn or T-bend, most of our lakes just aren't big enough to make a run long enough to charge anything.

http://www.stealth1charging.com/#:~:text=Stealth%201%20Charging%20ChargTech%20Industries,power%2C%20and%20Healthier%20batteries%20!!!&text=And%20here%20it%20is%20!!!&text=Stealth1%20gives%20all%20the%20power,the%20batteries%20still%20protecting%20them.

I’m just trying to come up with a solution for charging during an upcoming camping trip. Hate to buy a generator that I’ll never use

I'm no electrical engineer, but I'm thinking you would need more like 4400 watts for a 4 bank/10amp per bank charger.

40 amp output * 110v = 4400 watts

sounds like you would use it on the camping trip

whole lot better deal here.

https://mk.factoryoutletstore.com/cat/20875/Minn-Kota-On-Board-DC-Alternator-Chargers.html

Minn Kota MK-3-DC (3 Bank)
DC Alternator Charger

Can you just rent one from Home Depot?

Might be tuff to find a generator . Hurricane demand

Just suck it up and buy some 100ah lithiums lol

Your main problem is converted amperage. Your converting 12 volts to 120 volts. a 1:10 conversion. If your A/C charger runs 3 amps charging the 3 batteries, thats 30 amps being pulled off your single cranking battery. Thats why most solar systems use 48 or higher battery setup. Your trolling batteries are hooked up in series but charge in parallel. You would be better to get a DC charger or just hook your motor charger to each battery. But will wear down your alternator faster. I always wanted to make a device like a wind turbine that turned in the water while the boat was moving and would charge the batteries.