The inverter/Charger needs two power sources to get going. You have a 350A catastrophic fuse mounted in the battery bay (higher amperage if came with factory AGM). Check this to make sure it is good and NOT loose. Careful not to get a metal wrench in it and touch the metalic frame members nearby (spark/weld)
Then the AC power which comes from the Inverter breaker midway in the AC distribution panel (aka breaker box). Once these are present you can turn the charger ON. And arm the inverter...and Inverter will go into pass thru mode if both are available. You can remove DC and the Inverter will stay in pass thru...charger will quit. You can remove AC....and the contols will still work on DC the Inverter is previously ON will start inverting g and power the Inverter sub panel items (outlets, microwave, residential fridge).
If the 30A AC breaker is patent...and the 350A DC fuse is patent...then you may have to crawl I side the pass thru bay...look up find the inverter. And look for the button breakers on the casing to reset them.
If still no worky...you may have a failed component. So parts may be replaced...so it would be good to call Magnum to get their input.
Hope it gets you back up and running.