Air cooled or water cooled? I've owned several "RV" type air-cooled Onans (have 2 right now, a 6.5 in our motorhome, and 4 in our Party Cruiser)
Since it worked fine last year, fuel delivery is your likely culprit. You may have had some guck in the float bowl that's been sucked up in the main jet and causing it to run lean, or your fuel pump may be getting weak or going 'intermittent'. Do like OHB sez and open the main jet some, and see if it helps. Me, I'm leaning towards the fuel pump since it's totally dying on you...
Another thing to check is the automatic choke, make sure it's working ok...
1988 SkipperLiner 53x14
1995 Tracker Party Cruiser 32 *for sale*
2003 Chaparral 260 SSI
2000 Allegro Bus 40' DP