Sofomav

So I have built a Mk3 Cargo SSTO in order to go Duna. I can easily put the thing into Kerbin orbit and set up the maneuver to get a Duna encounter but the SAS system seems unable to keep the craft on the maneuver node when Im using the NERV engines. The only thing I can do is have the RCS on but I dont have enough of it for all the burns. The craft is very stable in the atmosphere and when re-entering.