It would help to have your data.
This approach will give you the starting index of the last string of zero values:
v = [zeros(1, 10) rand(1, 20) zeros(1, 5)];
vf = fliplr(v);
[~,idx] = max(vf > 0);
LastZeroStart = numel(v) - idx + 2;
I wrote it in several lines to demonstrate how it works.
