Billy, first off, the tile guy is right. According to the TCA Handbook, two layers of 5/8" plywood is recommended, and laying tile on this kind of floor even then comes with a cautionary note (i.e. it's not the best way to go). As far as the trusses are concerned, the 16" oc is good, sometimes they are spaced wider and that is a problem for tile. Again accordingto the TCA Handbook and ANSI 108, a floor that is to receive tile must exhibit a maximum deflection of no more than L/360 (i.e. 1/360 of th span, in your case 30x12/360=1"). The truss manufacturer can best answer whether his product meets this spec. under normal loading (usually 40 psf). If it were me, I would not accept any solution that adversely affected the utility of any part of the house. Good luck.