Radak: He ascended to the Ohel in which were the Aron and Mizbe'ach; it was elevated.

Rashi, from Sanhedrin 49a #1: His house was Hefker to Aniyim (Radak - he was very generous), like a wilderness.

Radak: Midbar can be a wilderness, which is totally unsettled. Or, it can be a place in civilization, but it is not cultivated; it is left for animals to graze; so it means here. He had a house there, and was buried there.

Radak citing Sanhedrin 49a #2: Just like a wilderness is free of theft ¹ , also Yo'av's house.

Radak citing Yerushalmi Makos 2:6: Once Yo'av was killed, Yisrael became like a Midbar (Korban ha'Edah - anyone could rule over them.. Pnei Moshe - the good things were removed from it. Yo'av used to build bathhouses, and feed Chachamim and their Talmidim.

Malbim citing Mahari: He had a house in the Midbar to feed guests and travelers. He was buried there to commemorate his good deeds of Tzedakah and Chesed.