You can buy a small, independent one (enough watts for your coffee maker) and plug your coffee maker into it directly or use a short good extension cord. Put it as close to the batteries as possible and use a large enough wire and it will be fine. From the web, it looks like a Keurig coffee maker takes about 1500 watts to run. Amazon has 1500 watt inverters for about $80-150 plus the wires and I would put in a fuse with direct wiring to the battery.
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