Check your voltage with a meter at the battery terminals without being plugged in next time it is moving slowly. Give it 5-10 minutes after unplugging to stabilize prior to measuring voltage after unplugging it. A fully charged battery should be reading in the 12.5-12.6 range for a 12v.
My guess is either your battery or your converter is going bad. If the converter is going bad, it may not be charging your battery fully even though it has been plugged in and charged up. It may be that your vehicle is charging the battery during the trip enough to make it operate smoothly but after camping a few days and being plugged in, the poor charge from the converter is showing in the weak movements.
Just a theory but one to check out.