it sounds like you are confusing VOLTS with AMPS... too totally different things.
if you are 'plugged in' to Shore Power, a 20amp outlet, then you have 120 volts of power. period. 120 volts is 120 volts. Items and devices you 'plug into' your rig, such as TVs and heaters work just fine on 120 volts, exactly as they are designed.
AMPS is how MUCH of this 120v power you are using. If you only have your TV on, then you may only be drawing 1 amp. Add a space heater, on low setting, and you might then be using a total of 6 to 7 amps. Add more items, and you get the picture. As long as all these items don't 'exceed' your 20amp Shore Power outlet's breaker, then you are perfectly fine. When the breaker does happen to trip, and you lose all power, then you'll know that you are using too much power for too many things at the same time.
AMPS is really a level of HEAT that the wires are experiencing while powering the devices and appliances in your RV. When the wires get 'too' hot, the breaker trips, giving you protection from using 'too much' at the same time. Smaller wires can only take so much 'heat', larger wires can easily take more heat, so 'upgrading' the Shore Power outlet from 20amp to 30amp, with larger wires, gives you more 'things' you can power at the same time.
Some of us have 50amp RVs, or motorhomes, which is a LOT more AMPS that we can use - 50amp power is different from 20amp and 30amp since it is actually TWO lines of 50amp power... more than 3 times the amps of a 30amp outlet. But that's probably not what your RV is designed for - yours is more=than=likely a 30amp coach, with a 30amp Main Breaker panel, and one roof air conditioner... or it may have two roof air conditioners, but limits you to using only one at a time, etc.
Electric HEATERS in the winter, and Electric roof AIR CONDITIONERS in the summer, are the largest AMP draws, and cause the most 'problems' with tripped Shore Power breakers. We all experience this. Managing 'what' you want to power is the key, but once you learn your RV, and what you can power when only on limited Shore Power, you'll be just fine. You may even have to turn some things off, while wanting to power something else, such as the microwave... at least while the microwave is running.