We still don't have a good way to deal with very small loads, when that is below the peak of the curve. In that case the efficiency spread is huge going up from near zero to the peak of 90 something.
https://www.google.ca/search?q=power+inverter+efficiency&biw=1093&bih=498&tbm=isch&tbo=u&source=univ&sa=X&sqi=2&ved=0ahUKEwjPrM6Ji83JAhVP0GMKHVaID2IQsAQIJw&dpr=1.25With the big inverter, a small load would be below that peak at poor efficiency, but then if you got a small inverter for the small load, you now need to wire the small inverter to the battery so it won't have voltage drop and so need more amps for the same watts and negate any efficiency advantage in amps.
Small inverters seem to all come with poor options to get good connections to an RV battery.
--Note the efficiency is measured by power in and out, not amps from the battery. Once again those watt things are out to confuse everybody!! :( We only care about amps from the battery that go into AHs when we are looking at this.
You could spend hours on a calculator trying to figure the efficiency difference between two sizes of inverters for a particular small load (as measured in amps) and how many more amps the small one would pull from high voltage drop poor connections to see if you come out ahead at all.
That 60w bulb I tested with is 2% of the 3000w's capacity so is way below peak, but the 60w is 40% of the 150w's capacity so is above peak. The 3000w still drew fewer amps to run that 60w lamp. it even did better with the CFL lamp.
How small of a 120v load would anybody want to run in an RV anyway, where it would be "worth" using a small inverter with little voltage drop at peak efficiency? If the load is that small, it can't be pulling many amps! So how many more amps is "twice" that? Some tiny amount is how many. So what does that come to in AH per day?
Always ask, "percentage of what?" EG my 3000w idle current is twice what the 150w one has. Holy Cow! Twice! Yes, it is 0.1 amp higher.
These little inverters are handy to have for sure. Plug them in anywhere and good to go. But let's not get confused about any "savings" in AHs in the RV by using one instead of the big one.
