how long of a 12 ga extension cord is safe to use

Thanks for all of the good advice. the good news is that the outlet is 4 feet from the breaker box
and is set up with an rv outlet to use a 10 ga wire. we never use the ac. and microwave at same time. all of the rest of the comments give me a lot to think about. thanks again!

Are you actually like second guessing if the OP is correct where the outlet is and camper goes??
Do you not think if there was a closer suitable outlet that they would use it?
Roflmao

100' of 30A RV cord has a 6V drop with a 30A draw. This is unlikely to be a issue for the OPs stated usage or most users unless the house voltage is low.

I would not recommend that anyone use a normal house plug for higher draw RV appliances. The typical 20A CB house plug circuit has MULTIPLE 15A PLUGS and some of those plugs may also be using power in addition to the RV. Using a dedicated 20A CB circuit and dedicated plug can be OK with closer management.

At the "side other house"
or
at the other side of the house?

Can you just plug into a regular outlet with regular plugs?

Your electric will work, but the voltage drop might be a strain on the AC that will burn it up after a while, or the cord might melt where the "gauge" is the least, at the camper outlet, or the house outlet...

What really is happening is relevant to the distance from the transformer at the pole, which is usually at the street. Power comes off the power line, to the street pole transformer, and then to the house. At the house power goes to the circuit breaker box, and then out to the houses outlets. Where is the circuit breaker box? How close can you plug in to the circuit breaker box?
If the box is on the camper side of the house and the 30 amp plug is at the far end of the house and you run from that , then you are running electric from the box, to the far end of the house, and then back to your camper.
It would be better to plug in the camper at the near end of the house, where the box is, and run there than to run the electricity to one side of the house and then back (adding another 100 feet to the string).
The house outlet is not the starting point of your issues!!! The circuit breaker box is (you cant really do anything between the box and the transformer, so just start at the box).

CA Traveler wrote:
STOP DO NOT USE THAT PLUG until you are absolutely sure that it's a correctly wired 120V 30A RV plug.

Excellent advice!

Some good info, some, not sure where all the fervor comes from...
OP, google has a plethora of online calculators to figure this out. It’s amazing, anyone can be a first year electrical engineering student with the click of a few buttons.
But the short story is 100’, assuming about 20amps (running ac + fridge) = 10ga min.
And that’s at the limit of acceptable voltage drop for that scenario.

Of course someone could have wired the plug wrong or insufficiently. I spoke that’s worth mentioning, but not getting snide over.

STOP DO NOT USE THAT PLUG until you are absolutely sure that it's a correctly wired 120V 30A RV plug. RVers that have plugged into one that was wired for 240V let their smoke out. And that includes ones installed by an electrician. The 30A RV plug is NOT a common household plug and installers make assumptions.

Buy 75' of 30A RV extension cords ($100+ on Amazon) and you will be OK based on your plan. Since the house wiring and voltage is unknown plug in a digital voltmeter and check the RV voltage as you turn on loads.

This sounds like a $600 mistake waiting to happen.

Before you plug in verify the 30 amp connector is 120 volts fed by a single breaker. Also check polarity. Notorious for being wired wrong.

Get a 100' extension. Check your running voltage the first few days. 108+ is fine. Some dip at start up is ok.

For best power get or make an extension with 30 amp connectors and #10 wire.

I have a total 110’ run from breaker to trailer using 10 gauge solid wire and the trailer’s 10 gauge cord set.
There is usually 120-122 volts with small loads and a 4 volt drop with the a/c running. I only tested the a/c once and didn’t see the initial drop as the a/c started. I have no need to run the a/c in the trailer at home as I don’t have any a/c in the house.
I have an EMS, so I really don’t worry too much about voltage drops.
I would not run any less than 30 gauge that far unless you are just going to have a small (10 A or so) load.