Yes ... 25-30 dB of gain should be about right for a vocalist. Set your channel trim there. Now press the solo/PFL button. What does the meter show? Now without touching any controls, unplug the receiver and plug in a mic in it's place. Now what does the metering show (in PFL)?
Do the audio LEDs (green) on the receiver seem to be bouncing normally?
Just to be sure ... you have tried with different mic cables and you don't have anything inserted in the channel, correct? Have you tried this on multiple channels all with the same result? Have you tried this on a different system? It is possible that you have a unit with hardware issues, but if it seems to work normally except for level it is not very likely the cause.
Feedback is not a function of the channel trim exactly. It is caused by "system" gain. In other words you could crank the channel trim wide open ... feedback will follow the output level from the mixer. So if you are getting feedback with the V30 but not with the wired mics it is because the V30 has more output into the system. In fact I did some recent testing between the mics we model (sm58 model is included in the V30) and our wireless had better gain before feedback than ANY of the wired versions did.