An inverter allows you to run standard 110v products in your truck. This allows for more options (vs. 12 volt products)and the 120 volt items generally cost less. Do you need one? Depends. What are you hoping to run in the truck and how large is the truck? For example, in my FL70 I have a 1750 watt inverter (1750 & 2000 watt are pretty standard for straight trucks), which I use to run a 110v refrigerator, microwave, laptop computer and a TV. My truck has 3 batteries and can support the system for a night before I restart the truck. When running several items at the same time, I always leave the engine running. You can add additional batteries and wire them in to run the inverter without running the risk of draining the primary batteries. You can also get inverters that will shut down when the batteries start to weaken. I have a marine battery charger wired in so that when I can get access to shore power, I plug in to recharge the batteries. When operating my 110v microwave, I will unplug the refrigerator, just to be safe.
There are some other inverters out there, i.e. ones that plug into cigarette lighters etc (@ 350watt). Be careful with these, in that if you draw more power through the wiring in the truck than it was designed for, you can cause a short or possibly a fire. If a larger inverter is installed, make sure that the wiring is sufficient to handle the load. You can buy cheap inverters (I would never plug my computer into one) or much more expensive ones. You will probably get input from other drivers about where & what to buy. I got my at Cabela's (Christmas gift certificate) and have been very happy with it. You might want to check out the internet. Mine has a switch for turning on and off and has two 110v outlets. I run a fused strip into the sleeper and plug into the strip.
If you are going to go the inverter route, you need to understand about batteries. I can offer a few tips. In the winter, the batteries recharge much slower off of the alternator than they do in the summer. The colder the temperature, the slower the recharge. Also, the more a battery is drained, the shorter its life. If you drain a battery too low on a cold winter night, you may need to get a jump and then possibly need to plug into a recharger to get the battery back up to strength.
I could not imagine not having the inverter in my truck. Good luck.
Drive Safe