I think many of the RNF are sincere. Sure they have hopes that it will soften you up for conversion but they think they are doing the right thing.
Most of the non-sincere ones would bother to help anyone inside or outside of the religion.
I regularly shoveled out the older people at my hall and also took care of one really old sister's lawn.
She insisted on paying me so I charged $5 for about 2 hours work. One time I tried to go up to $10 and she complained till I went back to $5 :-)
I don't think that type of thing really works to convert anyone. What it does do is give the doer a sense of pride that they are helping with the JWs good reputation.
I think being distressed over something is the primary reason for converting.