Let's say you plug your RV into a 30 amp electrical supply and you only use 120 volt appliances. You're using available amps from the 30 amp electrical supply for whatever 120 volt appliances are running, but the converter is drawing almost 0 amps because you're not using any DC accessories. It will use a small amount for items like the LP gas leak detector, clocks or maybe an aisle light, but not enough to really affect the amperage you are plugged into.
Your RV converter is rated for a certain amperage i.e. 30 amps, 45 amps, 55 amps. In other words a 45 amp converter is capable of running 45 amps worth of 12 volt
appliances in the RV. When your RV converter is working at its maximum capacity, which in this case is producing 45 amps for 12 volt
appliances and accessories, it is drawing around 5 amps out of the 30 amps available from the campground electrical supply.
Let's say you're plugged in and you're using a couple of 12 volt
overhead lights (2 amps) and a ceiling fan (4 amps). In this case your converter is drawing very little from the campgrounds 30 amp electrical supply. In another scenario let's say you're using a lot of 12 volt overhead lights (8 amps), you're running the furnace fan (11 amps), water pump (4 amps), 12-volt television (5 amps), range hood fan (2.5 amps), and the battery is being charged by the converter charger (3 amps). Now, when the converter is running close to its full capacity it draws the full 5 amps from the campgrounds 30 amps, leaving you with 25 amps for other 120 volt appliances and accessories. As you can see it's unlikely that all of this would be happening at one time. The bottom line is the converter amperage draw will fluctuate depending on the 12 volt demand placed on it.