I just now added an inverter. Towed it for almost a year and a half after putting in the residential frig without one. If you're going 6-7 hours, it's up to you. We found that at 6-7 hours, the frig went no higher than 42 and the freezer no higher than 10. If it's 8 hours or more, then you definitely need to.
What I decided to do was put in a 3k inverter and since I don't have a generator I tied the inverter into the Left side input of the generator transfer switch. That way I can power the whole left side slide if I want to for tv or anything else. I also added a third Group 31 Deep Cycle battery.
Vaughan & Tracy 2013 RW 36RL, 2013 F350 DRW CC, 2016 Focus. MorRyde IS, MorRyde pinbox, disc brakes, Comfort Ride Hitch. "Life is to be embraced, Grab it with both arms ! "