Inverters should always be placed as close to the battery as possible, using thicker cables. use 120vAC extension extension cords to reach the more distant appliance, never try and move the inverter closer to the appliance.
Most inverters will stop inverting when input voltage drops to 10.5v or less, some are 11v. The lower the input voltage the less efficient they become.
Basically the DC wires feeding an inverter need to be 10 times the total cross section of the AC appliance wires leaving the inverter. as ther eis 10 times the voltage difference.
Voltage is electrical pressure.
It is highly likely your rear 12v powerport uses wire no thicker than 18awg. If you were to try and power 300 watts through it, even with engine running and holding the battery at 14.4v, its likely that voltage at that plug would be 11v or less.
12v power port/ receptacles and their mating plugs are very poor electrical connections. Any device drawing more than 60 watts, needing to be reliable should bypass such a connector entirely.
I would not trust a distant 12v powerport to power a 300 watt inverter powering a 150 watt load unless it was fed 8 or 10awg wire, and perhaps not even then, especially if the device has a significant start up surge.
Also note that most inverters are modified square wave, and do not come close to a true sine wave. Lots of things will run fine, if slightly hotter on a MSW inverter, but some other things will either release their magic smoke instantly or will release that smoke very prematurely.
True sine wave inverters have come down in price in the last few years but they are still 2x the $, and some of these, put on an oscilliscope would have a joke of a sine wave.
If powering anything with a speaker, a MSW inverter can have that speaker release a 60 hz buzz.
Inverters are a great way to quickly deplete a battery, and while a high amp charging source seeking voltages in the mid 14v range, can quickly return a depleted battery to the 80% charged range, 80 to 100% takes no less than 3.5 hours, and ideally, for maximum battery longevity, it wants to live its life 100% charged, and be quickly recharged to as high a state of charge as possible after any significant depletion.
In short, do not trust 12v powerports, do not ask them to pass more than 60 watts for extended periods, and the further they are from the battery, the less that should be asked of them. If they have to be used and have to be distant from the battery, they should have no less than 10AWG directly from battery to powerport, fused at the battery. If going through this effort, one is better of using a quality electrical connector like Anderson powerpoles.
