Here is a very good reference for choosing and using extension cords.:
http://home.mchsi.com/~gweidner/extension-cords.pdf
The table on the last page is especially useful because it includes the expected voltage drop for a given length of a given wire gauge with a given current flow. The author recommends that you limit the voltage drop to 5% which in the case of a 120-volt feed amounts to 6 volts. I think as long as you observe this limitation, you shouldn't have to worry about damaging electric and electronic equipment in your RV.
I was pleasantly surprised to find out you can use a somewhat long extension cord and still be within the 5% limit. For example, you can be using a 75-foot, 12 gauge extension cord drawing 20 amps and the voltage drop will be about 4.8%.
One other thing to keep in mind is the construction of the extension cord. I find that all 12 gauge pre-built extension cords at Home Depot are only rated to carry 15 amps instead of 20 or 25. According to the sales guy, this is because of the coatings and coverings that enclose the 12 gauge wire. I think you might have to go to an electrical supply house or online to find a pre-built extension cord that is rated for 20 amps.
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2011 Keystone Cougar 24RKS
2007 Cadillac Escalade
Prodigy P2 Brake Controller
Past Trailer:
Forest River R-POD RP-175
Past Tow Vehicle:
2008 Toyota Sienna