Originally Posted by ArcticLyn
We tried a 3000W inverter today...
The wattage of the inverter will eventually make a difference, but when you tried it, what was it supplying power for? Others are correct in that it can draw a lot of power, especially considering the two devices that you indicate you want to run. Keurig will take between 800 and 1500 watts and the toaster is probably about the same. Be aware that is at least 125 amps of 12 volt. Neither of those loads is small, both would probably run the chance of topping out your 3,000 watts. The Keurig, depending on the model, will draw 60 to 100 watts all the time if you are keeping the water "ready for coffee."
Modern inverters are good, but they do require a lot of 12 volt to run.
What size wire did you use to connect the inverter when you were trying it?
What loads were plugged into it?
If you are trying to power the entire TT and have a converter you need to turn it off (breaker or unplugging)
It is true that when connecting loads to 2 parallelled 12 volt batteries, the positive should go to one of them and the negative to the other, but that is how they should already be hooked up to your unit.
if you drink a lot of coffee and love your toast, 2 -12 volt batteries really will not do you well for more than very occasional usage.