The reason I suggested checking power at panel is because it could be a bad main breaker (30 or 50???) -- it's unlikely but possible. I usually start isolating problem starting with easy stuff first that helps narrow problem area.
I agree with others that it could be the original transfer switch, but now that you've confirmed a large inverter, the possibilities are greater. The Freedom 3000 SW has a built in transfer switch and 150-Amp battery charger, so it's possible the installer placed it across the main line so it could power everything in motorhome (just not all at once). If so, it's possible that that transfer switch is not operating correctly.
Is it easy to check 110 Voltage at inverter in and out? What about 12-Volt output? Are batteries fully charged?
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