Figure out the size of the inverter needed based on the wattage you use in your RV.
Read the manual to figure out the maximum length of cable needed between the inverter and battery. This will vary based on the inverter.
Mount the unit in an area that is well ventilated and easy to get at. A good location is in the storage unit near the batteries.
Disconnect the negative battery cable. Wait five minutes for any charge to disappear before proceeding. Connect the inverter's ground wire if it has one.
Run the cables from the inverter to the battery. If holes have to be drilled, add grommets to them so the wires don't rub.
Crimp all connections and reconnect the battery.