Do regular adapters for USB work in both 12V and 24V car systems? Would they work with 28-29V (when alternator is running)?
Most vehicles I've dealt with running a 24v system do so by doubling up with two separate alternators and (at least) two 12v batteries ran in series. Only the starting system uses the 24v while the rest of the vehicle runs off of 12v. They do this so they can run 12v accessories and lights and not re-invent the wheel. In order for it to work they only draw power from one battery. Note: Semi-trucks may be setup differently, using only one alternator producing 24+vdc, but accessories will still most likely be only 12v.
The reason I mention this is, more than likely the cigarette socket will only supply 12v. It is easily checked with a multi-meter. I would suggest your adapter would work just fine, but check the voltage to make sure.