Congrats on the new unit whatever it will be. In defense of the salesman an inverter would not be something considered necessary for dry camping. It is a nice to have but certainly not a need to have. The RV is designed to operate using 12 volts and propane. However if you decide there are some things that run from 120 volts you feel you need for camping then sit down and make a list of what they are. The size of the battery bank, inverter and possible solar system will be determined by that list. There are 30 dollar solutions for simple needs all the way up to 5000 dollar systems for more elaborate needs.
Happy camping.