memenuhi also can. But, mengisi is better because they just fill their bags with bananas and peanuts. They do not fill their bags with it until full because memenuhi more to full something because 'penuh' means full.
Use the plural form when you would otherwise assume the noun is singular. If you read "Saya mencintai kucing saya", (I love my cat[s]) you would think of only one cat. But with "Saya mencintai kucing-kucing saya", (I love my cats) there is more than one cat.