Just keep it simple. I would send it to the secretary but copied to the treasurer and strata manager, addressing it to the committee as a whole asking for an explanation from the committee. The committee could instruct the manager to respond. Alternatively, a role of the committee is to supervise the treasurer. The buck stops with the committee and you will have ensured they get it addressed to them.
I would also give the committee more than 5 days so that you give the committee sufficient time to arrange among themselves to meet to discuss and decide their response. Ultimately, you want to be able to demonstrate at the Tribunal, should it come to that, that you were utterly reasonable at every step and gave them ample time to investigate and respond.
As JT suggests, keep it simple. You just want to know what those particular transactions (state the amounts and dates) were about: from whom, to whom, for what purpose. Don’t go into any theories of your own or airing of other grievances. Keep it simple. At most you might note that you had made previous enquiries on particular dates.
You could conclude by stating: ‘I intend to take the matter further with appropriate authorities if I do not receive a sufficient explanation by (some date, say 3 weeks later).’