I am far from an expert on inverters and their efficiency. However, to add to the mix I will offer what we have on our coach. These were installed by the previous owners (along with a solar system) who enjoyed dry camping. We haven't changed the system and do relatively little dty camping, but have been pleased with the system they had installed. I'll leave it to others to review the manufacturers' documentation and determine whether or not the mix of small and large inverters is a good solution.
Our large inverter is a Magnum,
2800W 12VDC Pure Sine Inverter Charger MS Series | Magnum Dimensions (https://www.magnum-dimensions.com/product-inverter/2800w-12vdc-pure-sine-inverter-charger-ms-series)
while the one we use for our TVs and other small loads is a Morningstar SureSine 300,
SureSine - Morningstar Corporation (https://www.morningstarcorp.com/products/suresine/)
Four separate issues when evaluating an inverter or inverter/charger:
Is is sufficiently large (in watts) to handle the loads, including starting loads of what you will be running on it?
What is its draw in amps or watts (remember amps times volts= watts so either amp or watt draw can be converted to the other for comparison) in standby-mode (i.e. when no 120 VAC items are being run)? For most residential refrigerator- an important consideration, as close to 50% of the time the refrigerator will not be running.
What is its efficiency when running the loads you will be running (i.e. load once started, not the momentary load when starting)?
Cost?
Your magnum is 90% or over efficiency at 300 to 700 watts. The smaller one is 90% at 100 watts. 85% at 300 watts.
One amp less draw at 300 watt output to use the Magnum. Seems to be 24 amp hours per day?
Minus the power loss from the extra cabling.
Both have a chart showing their efficiency.
A Victron at low power levels maybe even more efficient as it has a smaller transformer than the Magnum
Some put small inverter close to batteries, in my case under bed , and use the standard fwd 110 electrical wiring and outlets to run electronics. So no loss from cabling. The SureSine mentioned above only seems to use about 5 watts when on with no load. So perfect for leaving on all the time to maintain all the memory in your electronics.
There is no "power loss from the extra cabling." The small inverter is mounted as close to the batteries as is the large inverter, and there is large cabling to the small inverter.
cabling aside. As Brett said the inverter needs to match the load at its max efficiency point.
The 500-1000 watt loads the Victron may well save at least an amp per hour.
At higher loads and internal temps it's limitations would be the possible derating that could occur under heavy loads most here would rarely do.
Learned a lot on this.
If the max efficiency was paramount the special construction of the Victron would save a lot of battery power meaning less gen run time.
Need a chart to verify the actual efficiency at the intended load
I have been full timing off the grid for 15 years. The only inverter I had for 10 years in my 1981 Foretravel was a 400 watt modified sine wave inverter. I've been in this 1993 Foretravel for 5 years and use a 750 watt Schumacher that costs $45. I have an identical spare on board in case I ever need it. I have no idea how efficient these small inverters are but they do what I need to do and I have plenty of battery left at the end of the day even in the short days of winter.
I use the generator if I want to use the microwave, pizza oven or AC.
I think a significant part of the overall efficiency in the real world would include the power consumption when inverting with no load. According to the manufacturers' data my large inverter consumes 30 Watts when inverting with no load, while my smaller inverter consumes about 6 Watts. That appears to me, at least, to be a rather significant difference--particularly if it's powering a load that's not on continuously.
Had it in another post but if your Magnum has the BMK monitor you can look at the amps it;s using and know exactly,if you don't
have the BMK would sure get it,easy to install.