If you are just using the extension cord to maintain your batteries and run your fridge (on electric), the 14 gauge extension cord will be just fine, no worries.
The three biggest draws in your camper are AC, Microwave and the water heater when it's on electric (that's a huge draw). I regularly have to plug into standard, 20amp outlets that are too far for my heavy, 30amp cord(s) to reach (in addition to living in my 180 pretty much full time it is also used as a production studio for our broadcast business). I also carry a 50', 12 gauge 20amp extension cord (about $40 on Amazon) for these occasions. Here's the real world scoop:
On a 20amp you can successfully, and safely, run any one of the "big three" along with everything else in your camper, at the same time. This includes fridge, laser printer, three laptops, production computer, water pump, all the lights and a Keurig coffee maker. If you're running the AC, turn it off to Microwave or fire up the water heater. In reality, you can run the AC and microwave at the same time on 20amp as long as your timing is good enough that you don't accidentally fire up the microwave at the exact moment your AC compressor is cycling on.
On a 15 amp breaker, it's any one of the big three and less of the others. If you're NOT running one of the big three, you can power everything in your camper at once off a 15amp, including the fridge.
The only time we've kicked a breaker on our feed was an AC/Water Heater combo on a 15amp outlet when I forgot to flip the water heater switch off electric. It's important to note; if your water heater is flipped to electric, it will start to heat as soon as you plug in. The toggle inside on your panel is only for the propane heat. Also, even though I know a lot of people on here do it, you shouldn't use the breaker in your camper as an on/off switch for your water heater. They are not "switching" breakers and the constant toggling will weaken them over time.
A bit more than you asked for but I see a number of questions about this stuff and thought I'd throw it all out there. Other people may have different experiences.