"“When they had made an end of dividing the land,” and all the tribes had been allotted their inheritance. Joshua presented his claim. To him, as to Caleb, a special promise of inheritance had been given; yet he asked for no extensive province, but only a single city. “They gave him the city which he asked, ... and he built the city, and dwelt therein.” The name given to the city was Timnath-serah, “the portion that remains”—a standing testimony to the noble character and unselfish spirit of the conqueror, who, instead of being the first to appropriate the spoils of conquest, deferred his claim until the humblest of his people had been served. PP 515.1"
"According to the word of the Lord they gave him the city which he asked, even Timnath–serah in mount Ephraim: and he built the city, and dwelt therein."
EGW directly quotes Joshua 19:50.
The cities of refuge were so distributed as to be within a half day's journey of every part of the land. The roads leading to them were always to be kept in good repair; all along the way signposts were to be erected bearing the word “Refuge” in plain, bold characters, that the fleeing one might not be delayed for a moment. Any person—Hebrew, stranger, or sojourner—might avail himself of this provision. But while the guiltless were not to be rashly slain, neither were the guilty to escape punishment. The case of the fugitive was to be fairly tried by the proper authorities, and only when found innocent of intentional murder was he to be protected in the city of refuge. The guilty were given up to the avenger. And those who were entitled to protection could receive it only on condition of remaining within the appointed refuge. Should one wander away beyond the prescribed limits, and be found by the avenger of blood, his life would pay the penalty of his disregard of the Lord's provision…
"And among the cities which ye shall give unto the Levites there shall be six cities for refuge, which ye shall appoint for the manslayer, that he may flee thither: and to them ye shall add forty and two cities."
Both passages discuss the cities of refuge and their purpose. While EGW provides details not explicitly stated in Numbers 35:6, the core theme is present.
"Thou shalt prepare thee a way, and divide the coasts of thy land, which the Lord thy God giveth thee to inherit, into three parts, that every slayer may flee thither."
Deuteronomy 19:3 speaks of preparing a way for the slayer to flee to the cities of refuge. EGW expands on this by describing the roads being kept in good repair and signposts being erected. This is a direct application and amplification of the principle in Deuteronomy.