Wow. I have been studying philosophy for half a century (and in fact am planning to write a book on the subject), and I have never heard anyone claim that materialists don't believe in truth because they don't believe in non-material objects. That is one damn strange argument.
The fact is, materialists have a different understanding of the correct meaning of the term "truth" than that. Ditto the pragmatists.
The fact that you don't seem to be aware of that indicates to me that you have a quite poor understanding of Western academic philosophy.