Fuses and circuit breakers are always good to check. The panel in our 2552 is under the passenger-side bunk.
The surge protector's display panel should give you useful information if you're plugged in to shore power. In our 2552 the panel is mounted on the wall behind the driver's seat. If you have the surge protector Phoenix currently uses, made by Progressive Industries, it should alternately flash a status code ("E0" means no errors - good), the AC input voltage, and the AC current being used. If it's dark, you've lost shore power.
But the coach batteries should power your lights and inverter without shore power, unless they are completely discharged. The little black status panel that shows how full your holding tanks are, also on the wall behind the driver's seat, has a "BATT" button that shows battery status. Check that. If the coach batteries are dead, you'll need to figure out why.
Tell us what you find. The electrical system can be confusing at first.