Matlab - Using findpeaks with fvtool
fvtool is interactive so it doesn't let you recover an data in a script. Fortunately you can use the freqz function to get the same frequency response. Something like this might work:

[h,w] = freqz(b,a);  % h is frequency response, w normalized frequency
mag = 20*log10(abs(h));
[minval,minidx] = min(mag);
minfreq = w(minidx); 
[maxval,maxidx] = max(mag);
maxfreq = w(mcxidx);