New ones don't come with an inverter either. They do have dual (AC/battery) power systems and a converter/charge controller that runs the 12v lights and appliances such as fans off the AC when shore power is available. The converter will also have a charger that charges the battery when plugged in to shore power.
An inverter lets you actually run AC devices off the battery. You can purchase a small one to allow you to plug in cell phone chargers, iPads and the like, but if you want to run a large inverter capable of powering something like a small microwave you need lots of battery! I have a 1500 watt inverter in my TT but I also have a 4D AGM battery that is a nominal 210 AH. Even so the only thing I run regularly is the HDTV.
My first TT was a '53 Aljo 13 footer. The only "power" it had was propane. Everything ran off that. The lights, small reefer, etc. Didn't even have a battery.
'91 F350 pulling an '09 Pacific Coachworks Tango 276RKS