Learn Turkish: Kimyon çuvallarına doğru yavaş yavaş yürüdük. - We walked slowly towards the cumin sacks.

This is an ikileme (lexical reduplication). Repeating an adjective like yavaş (slow) is a very common way to create an adverb of manner in Turkish. While yavaşça also means "slowly" or "quietly", yavaş yavaş specifically emphasizes the continuous, gradual nature of the action, giving a strong feeling of "little by little" or taking one's time.

Let's break down the suffixes on çuvallarına. Why is there an -ı and an -n- in the middle?

This happens because of the noun compound kimyon çuvalı (cumin sack). To make it plural, we add -lar, giving kimyon çuvalları. Next, the word doğru (towards) requires the dative case (-a). Whenever you attach a location or direction suffix to a compound noun's possessive ending, you must insert the buffer letter -n-. That gives us root + plural + compound marker + buffer + dative: çuval-lar-ı-n-a.

I thought doğru meant "true" or "correct". Why does it mean "towards" here?

You are right that doğru means "true" or "straight" as an adjective. However, when it is used right after a word with the dative case (-a or -e), it acts as a postposition meaning "towards" that object. You can spot this grammatical structure here because of the -a at the end of çuvallarına.