The two most common issues with partial power loss are tripped GFIC s and loose connections in the 110 circuit panel .
My coach has 2 GFIs and most all other outlets are daisy chained through them. So look at all your outlets , trip and reset any GFIs you find.
My microwave has a dedicated circuit breaker in the main panel.
First , I'd pull the microwave cord from it's outlet and test for power. Then if no power to the outlet, disconnect the coach from shore power , pull the face off the 110 panel and check all the screws that hold the wiring to the circuit breakers. Take your multi meter and test for zero ohms through the breaker, if they test ok then place the face back on and power up the coach. If still no power to microwave, particularly if you have a circuit breaker marked for the microwave , turn it off and see if any other outlet goes dead, if you have a dead outlet between the micro and the circuit panel then that outlet box will be the wiring junction, and then, you will need to pull the outlet and check connections.
EDIT on re-reading , I see that you said all breakers are hot , sorry I missed that.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.