Here's what I think on extension cords. I'd start with investing in a good quality one.
Get one in a single length if possible to avoid using several shorter ones to make up your 75' length. Every plug/connector connection is a potential source for adding resistance and increasing voltage drop, and overheating (see below). Standard lengths are 50 & 100' so better to use a 50' cord if you can.
Get a cord that is UL/CSA listed. Extension cords are not required to have UL/CSA ratings. If not "listed", a manufacturer can make any claim they want about them and labels like "extra heavy duty", "commercial grade", "durable", etc. can mean nothing. There are a number of different insulation jacket types as explained
here.. The best extension cord (besides the time & money) would be to buy the cord with the jacket type you want and then add your own commercial spec-grade plug & connector. You could do a custom length this way too.
Always turn power off before plugging in. If you don't, there is an inrush current caused by the converter/charger that will cause pitting and attract dirt on plug & connector contact surfaces and eventually lead to an overheating connection. In the beginning, I had two (cheapo ones) go up in smoke where I plugged the 30/15 adapter into the extension cord and I only had max converter/charger + fridge running. Keep all your plug blades clean with sandpaper as needed.
A fridge on electric has a 300 watt element and will draw less than 3 amps on 120 volts. But if the converter/charger happened to be in charging mode (like after using elec. jacks or not kept plugged in) you could be drawing 10+ amps. A cord should be min. 12 gauge for 50-75'. On voltage drop, you also have the length of wire from the house's receptacle to the main panel (*could* add another 50-100'), plus extension cord plus the shore power could and you might have 200' or so of overall wire. A #10 cord would be better but would be a pain to lay out every time just for the fridge.
A permanent voltmeter inside your TT is a very good "tool" to have to keep an eye on voltage. Could be plug-in or hardwired and LED type is better. I would never camp without one since CG voltage can vary a lot and sometimes be dangerously low (below 102-104 volts).
On charging, if a WFCO converter/charger, they are known to sometimes fail prematurely and can even damage batteries (happened to us). Over the winter I would take the battery(s) inside and keep it/them on a maintainer. You want your converter/charger running all the time otherwise there are parasitic loads that will discharge the battery in a matter of days. If you can't plug into 120 volts, disconnect the battery (some add a battery switch).
