I use several 12v to USB chargers. Two are plugged into the cigarette lighter (power ports) in the camper and a couple more in the truck. IPods, phones, Bluetooth speaker, FRS radios, InReach Explorer, BT headsets are all charged by USB power.
You're correct that it is wasteful to convert from 12v DC to 120v AC, then back to 5v DC. Touch the DC-AC inverter and AC-DC converter when operating. That heat is evidence of the waste. A Trimetric could tell you how much waste.
I also use both for those times that staying in a motel is called for or if in an all day seminar, etc. Fortunately, all the power adapters and cables fit in a gallon ZipLock and weigh little. My phone came with a power unit that has folding AC prongs plus a DC power port plug in the same unit.
Paul