Numbers 18

"Then the Lord said to Aaron: 'You shall have no inheritance in their land, nor shall you have any portion among them; I am your portion and your inheritance among the children of Israel.'" v.20

Read chapter 18
The Lord is their portion.  Makes you think of the well known Psalm 23 when David says "The Lord is my portion, I shall not want."   
"The Levites had no inheritance of land and fields to work with from the LORD, but instead they were to be given the tithes of the rest of the sons of Israel as their inheritance. With twelve tribes presenting them with a tenth of what they produced any year, if we assume the tribes are equal sized (of course, they were not really, but just as an assumption,) each individual Levite would actually end up with 120% of the produce of the average Israelite. God was making sure His ministers were well cared for.  So the Levites too were to offer a tithe, and their tithe was to go to the priests of the family of Aaron. The rest of the tithe was theirs as the equivalent of what they would have gotten from the threshing floor or the winepress, had the LORD given them lands, as He did to the rest of the Israelites. Thus the Levites were taken care of physically, so that they could concentrate on learning of the LORD and His ways to teach these things to the rest of the Israelites."(Bible Teacher Nathan Johnson)
Even as the Israelites received tithes they were to tithe from the tithe.  It's like when you get a gift do you give the best part of your gift to someone else.  So instead of the Levites comparing and complaining that they aren't like the other tribes the Lord provides for them in a different way.  Would you rather have your portion be land or the Lord?