Maybe. You'll need an adapter on the RV end of it to fit whatever your service cord uses, and you'd be limited to the 1875W of the outlet you use. I use a cord much like this for our little camper, but it only has a 5000BTU AC unit and a small refrigerator, so it works fine. If your RV has a lot more or bigger appliances, you may be limited in what you can safely run at the same time. Be careful, as you may exceed the rating of the outlet before you blow its circuit breaker (or fuse).
With precautions, yes. Remember that the junction will not be at all water-resistant; pulling it across a wet lawn is a bad idea unless you've thoroughly sleeved and/or wrapped it. You'll probably want to tie an overhand knot there for strain relief so it won't pull apart. The current-carrying capacity will be based on the thinnest wire (the 50' version of the same cable should be the same gauge). The junction will add a tiny amount of resistance. It won't be much, but could create a hot spot if you are at the current limit for a single cord.
This product is made in Vietnam .
Yes it is, it is heavy duty. I needed it to work in the warehouse and it serves its purpose.
a 12GA cord is more than sufficient for 300W; that's less than 3A, and 12GA is used for household circuits of 20A. Note that many outlets are limited to 1850W, but that's a non-issue here. I'm assuming, of course, that you did not intend to type "3000 watt heater." Not only would that exceed what the outlet could safely handle, that would be 25A which exceeds the rating for 12GA wire; you'd need 10GA for that, and wired directly rather than plugged into an outlet.
Most likely because outlets tend to be rated for a maximum of 1850W, which is less than 15.5A.
Yes, it can. Very durable.
No, it is made in Vietnam.