STOP DO NOT USE THAT PLUG until you are absolutely sure that it's a correctly wired 120V 30A RV plug. RVers that have plugged into one that was wired for 240V let their smoke out. And that includes ones installed by an electrician. The 30A RV plug is NOT a common household plug and installers make assumptions.
Buy 75' of 30A RV extension cords ($100+ on Amazon) and you will be OK based on your plan. Since the house wiring and voltage is unknown plug in a digital voltmeter and check the RV voltage as you turn on loads.
