To be fair to the sales staff, you don't NEED an inverter.
Everything in the RV should be able to operate from battery power other than the tv, microwave and A/C unit. Even rigs I have seen with residential refrigerators come with an inverter dedicated to the operation of the refrigerator.
The problem is that you want luxuries, not necessities. For that you need an inverter, lots of battery power, and then a way to recharge those batteries.
Fortunately everything you need to know is in the thousands of threads on this forum. Get a drink, get comfortable, and start reading, you have a lot to learn.