I live in a manufactured home and have a central heating/cooling unit outside that comes on when I turn it on at thermostat, but there is no warm/cool air blowing through the vents and nothing seems to being sucked through the filter. Have ensured filter is clean and ductwork underneath home is intact. What else could it be? Thanks!

You said it "comes on". What "comes on"? Switch the fan switch to ON. Does the air flow now? If no, you have a motor issue, possibly a start capacitor. If yes, you could have a t'stat or fan switch issue. Let us know what happens.