A 30 amp extension is certainly fine, but still some bucks. If you are only keeping the inverter/charger on or a dehumidifier, or the like, you can save $ using a 12 guage 15 amp. Additionally, it is easier to handle and store than the bulky 30 or 50 amp cords.
I occasionally use a toaster oven or the microwave, but not both, with my 25 ft. 12 guage. Its 15 amp current capacity flows with a whole lot less voltage drop than a 14 guage. It's great for when the coach is in storage, and can handle minimal appliance use prior to a trip or if used for overnight guests. Just don't kick on an AConditioner.
Joel