300W inverter will not run any drip coffee maker that I know. The Keurig, by reports, draws in excess of 1500W. Others will probably be over 1000W.
To distribute 120v through the inverter, there are basically three choices: 1) Plug camper's shore power cable into inverter or dedicated/connected 30 amp receptacle, 2)use a transfer switch that will provide 120V to either select circuits or the entire existing 120V load center, 3) install dedicated receptacles connected to inverter. In methods 1) and 2), the onboard converter should be interrupted from 120V. In a truck camper, it may be possible to locate the inverter in a central location where power is taken directly from the inverter.
