I run a cord from the inverter to the main power hook up. Where I hook up in the campground. Then, I make sure the 120v circuits I do not want to use are turned off in the circuit breaker box, to save my battery.
I am not sure what you want to run with 3500 watts? That would run electric water heater, microwave, toaster, and a few other, small things at once! Unless you have about 1000# or more of battery, you will run them down in about 15 minutes! 3500 watts is a HUGE draw for a normal battery bank, even of 6 big batteries.
I would suggest NOT using that much. Space out your use, do not use everything at once. If you need anywhere near 3500 watts for more than a few minutes a day, you need to either plug in to 120, or run a generator.