when plugged into 15amp power, via a regular household outlet, it's easy to have many things in play without realizing it, such as high draws from the Battery Charger within the Magnum panel, the Electric water heater, if left on, or even the engine block heater switch(mine is on the side of my bed, which leads to easily and accidentally flipping it on!)...
while the Inverter can remain ON, even when you are plugged in, it is really only INVERTING when you DON'T have power - it otherwise does NOTHING when you do have power, so it should not have any impact on whether your outlet is providing power, or not. You could easily cut OFF the inverter, but it would actually make no difference on your shore power, but all your 120v power would also remain off when shore power is lost. Probably doesn't matter unless you have TV satellite receivers that must 'reset' every time this happens, etc.
If your inverter is going into 'fault' mode, it's likely not about shore power, but about the internals of the inverter/charger having issues with the battery connection, or the battery LEVELS. If your battery levels are low when you first plug into 15amp power, the Magnum charger may be trying to charge them, tripping it's fuse/breaker, but then resetting automatically, which mine has done also from time to time. The small round 'button' fuse breaker on the Inverter may not always automatically trip, but sometimes can miss power for a moment, then continue providing it the next, when it's right on the border of tripping totally. You may see a loss of power to 120v outlets and items only for a moment, such as a satellite receiver 'reset', for seemingly no reason, while everything else seems to continue working as normal. When it trips fully, though, you'll have NO 120v power from the Inverter, and you must then push in that fuse/breaker to reestablish Inverter AND pass-thru 120v shore power.
Because these systems are somewhat complex, and they are all 'interwoven', it can be a puzzle sometimes as to 'what' has actually lost power. I've run around on several occasions checking breakers in the main panel, breakers at the shore power, and even my battery wiring, only to find out it was the simplest of things - the small tiny tripped Inverter fuse/breaker - push it in and all is good again!
(this has typically happened when we overwhelmed the Inverter's pass-thru 20amp fuse/breaker by having too many electrical items plugged in and operating while even on shore power - since the outlets are ALL thru the inverter - several small space heaters on 'high' can easily do this.
two Campers, two Electric cars
former diesel pusher traveler