power loss between 12v to 5v ac inverter vs. 12v to 5v dc only inverter
You are drawing very little power from an inverter capable of much more so it's not surprising the efficiency would be abysmal. 

The kind of adapters (12V->USB) you mention are typically non-isolated DC-DC converters. They would likely have pretty good efficiency. Many newer vehicles have USB outlets built in to the dash. 

One caveat is that cheap 12V-USB converters might well have no overvoltage protection, so a single-point failure could put 13.8V (vehicle supply voltage of a '12V' car with motor running) right on the USB jack of your tablet, almost surely destroying the tablet. That's a lot less likely with an inverter- usually if they fail the voltage goes to zero. 

Here's a typical buck converter circuit capable of 500mA with 25V in (maybe not as much with 12V in): 



In operation Q1 switches on and off at perhaps 50-100kHz, staying 'on' and 'off' just long enough to maintain the proper output current through L1 at the proper output voltage. That's controlled by a feedback circuit that maintains a certain 'on' time and adjusts the off time to maintain 1.25V at the output of the voltage divider R1/R2 (meaning the output voltages is nominally 1.25 * (R1+R2)/R1 = 5.0V). Because the inductor (ideally) has no loss and (ideally) the switch Q1 and diode have no losses the losses are all due to 2nd order effects- switch/diode voltage drop and inductor resistance and core loss, mostly. So efficiency is ideally 100%. In practice, maybe 70-85% at moderate loads. A linear 5V regulator has an ideal efficiency of 36% with 13.8V in. 

Should Q1 fail short (perhaps due to a transient on the 12V bus with poor protection in the converter) or there be a bad solder joint or crack on R2 then the output will rise to much over 5V. 

There are better more modern DC-DC converter circuits, this is just one that is sometimes used in inexpensive converters. 

Personally, for an expensive tablet, I would seek out a device sanctioned by the tablet manufacturer (for example, one sold in the Apple store for an iPad). That way it's most likely acceptable quality and you have some recourse if things go badly.