Extension Cord

Just for a sample of power use per item in my TT:
Fridge 2.733 amps
BR fan .33 amps
Microwave 10.25 amps
AirCon 12.05 amps
Converter at idle .117 amps
Water heater 10.38 amps
Toaster 5.77 amps
Space heater 1500 watt 12.5 amps
Electric griddle 12.5 amps
Single serve coffee maker 6.67 amps

Granted, not all TT have the same appliances but this will give you an idea. Hopefully, the receptical and circuit you are plugging into is 20 amp vice 15. IF the supply line is all 12AWG to the receptical it can use a 20 amp circuit breaker per NEC.

Seperate issue. If you are going to be in the TT for an extended time, look into getting a 100# propane tank. Depending on your heating and cooking choices.

What about water and sewer in your situation?

Wow, that 100' 10AWG extension cable is costly! But it would cut your voltage loss some.
I measured 107.7VAC on my EMS at 110' of 12AWG from my breaker box to the TT. Starting voltage, no load 122VAC. This was with just the AirCon on and converter, all else turned off. Pulled 12.9 amps.
Added in the fridge and it read 104.5VAC pulling a total of 15.8 amps.
I installed a Progressive Industries EMS 30 but used a Kil-A-Watt meter prior. The Kil-A-Watt cost about $25 at Harbor Freight. It's good to know how much power your RV gear pulls. In volts, watts, & amps.
Bottom line, keep your cables as big and as short as possible and what you can afford.
Www.calculator.net voltage drop calculator. Plug in your wire length, AWG, starting voltage, and amp load and it will tell you expected voltage loss. Very accurate. Most electrical and electronics are designed run no lower than 105VAC.
You'll need to educate yourself and others on what can be turned on at the same time given your power source. With 100' 12AWG extension cord, no microwave with AirCon OR electric space heater. Or any electric heated cooking device including coffee maker, toaster, frypan, etc. One of those at a time!

Even though you pan on using minimal power except for a heater and especially because of the heater, I would suggest using a 10 gauge extension cord. Be prepared for connector and trailer 15/30 adapter failure on any cord you decide on. I would be prepared with a 30TT female connector to replace the standard 15 amp cord body for the trailer end of cord.
another option is to get 10/2 with ground UF cable instead of an extension cord. Build your own "cord" with it.
10/3 100foot extension cord

Good to go. Worst case the breaker will trip and you will need to adjust the load in the RV or discover the shared load at the source.

I think that would work fine. I found a 100 ft 12ga. cord at Costco for a good price.