Where Rav Elazar Menachem Shach and Rav Isaac Zev Soloveitchik disagree about the seventh year in Israel seems to be in this: Rav Soloveitchik holds land that was conquered by Jews coming out of Egypt but left by Jews coming back from Babylonia is obligated in all obligations of the land of Israel. And when it says They did not sanctify the land that means so the poor could have the fruits and vegetables that grow by themselves on the seventh year. Rav Shach does not like this. To him those lands are obligated also in tithes to the poor --not just ספיחים things that grow wild.
The way he gets to this like this: We have a Halacha to Moshe from Mount Sinai that Amon and Moav have to give tithes to the poor on the seventh year. This Rav Shach understands to be relevant to a time that those lands will be part of Israel from the Torah. That means to say that even in the future when that land will be part of Israel and obligated in all obligations that Israel is obligated in Trumah and Maasar etc still they will be required to give tithes to the poor in the seventh year. And also when such land is not part of Israel, still they have to give tithes to the poor because of a decree. From this we can understand the Gemara in Yevamot 16a. עמון ומואב מעשרים מעשר עני בשביעית דאמר מר הרבה כרכים כבשו עולי מצרים ולא כבשו עולי בבל דקדושה ראשונה קדשה לשעתה לא קדשה לעתיד לבא והניחום כדי שיסמכו עליהם עניים בשביעית
Amon and Moav gives tithes to the poor on the seventh year because many cities were conquered by those coming up from Egypt, but not by those coming up from Babylonia, and they left them so the poor would be able to depend on them in the seventh year.
That is the areas of עולי מצרים are required to give tithes to the poor on the seventh year. That is a proof that Amon and Moav also do so.
I wanted to point out that according to this way of understanding the Rambam Trumah I:5 you don't need the land to be obligated from the Torah. And we know that that is good because if we would need that that would conflict with what the Rambam says in the end of the chapter. This way we have the basic step of a halacha le'Moshe MiSinai and then a decree כעין התורה like the Torah that just like Amon and Moav would be liable in all obligation but added to that would be tithes to the poor so land of Israel proper but not conquered by עולי בבל has the same law because of a decree.
The main point that Rav Soloveitchik has I think is the fact that in מתנות עניים we doing find that the land of עולי מצרים is obligated in tithes to the poor. He only mentions Amon and Moav. On the other hand there are a few points that bring support to Rav Shach. One is the Gemara itself in Yevamot 16a. If we read it like Rav Soloveitchik it is a bit strained: "Amon and Moav give the tithe of the poor. How do we know this? Because the Jews coming up from Babylonia did not sanctify many cities so the poor would have support from them on the seventh year." According to Rav Soloveitchik what support do the poor have on the seventh year? Only the ספיחים what grows by itself. And when they returned there was no decree on the ספיחים anyway!
The wording of the Rambam is ולא פטרום. The idea in itself seems to be relevant to the next Halacha in the Rambam where we have that Beit Sh'an and Ashkelon were פטרed- left to be not obligated. So here the Rambam is saying in opposition to that that the area of עולי מצרים they did not פטר.
