1) This is just a first glance at a subject about the borders of Israel and presenting a question on the Rambam.
2) It is generally understood that קדושה ראשונה קידשה לשעתה ולא קידשה לעתיד לבא. The first sanctification sanctified the land only for its time and not for when Jews went into Exile. And קדושה שנייה קידשה לשעתה וקידשה לעתיד לבא. The second sanctification sanctified the Land for its time and for all the future.
But there is an option that the first sanctification sanctified it permanently. We will see Rashi has to use that option to explain and mishna in Sheviit.
3) Sichon and Og were kings that attacked the Jews and lost the war and their lands. This area is beyond the Jordan River. It was settled by Jews coming out of Egypt. According to the grandson of Rashi all or part of it was settled by Jews coming back from the first exile in Babylon.
But to Rashi none of it was settled by Jews returning from Babylon.
Tractate Yevamot 16a
To Rabbainu Tam the land conquered by the Jews coming out of Egypt does not determine the borders of Israel today.
To RT if an area was settled by עולי בבל then it has the holiness of the Land of Israel and is חייב in ביעור in the seventh sabbatical year. In his first answer that land is the part of Sichon and Og that was the original land of Sichon. But land that Sichon conquered from Moav [in the Torah] was not settled by Jews coming back from Babylon. And so the parts of Sichon that were conquered by Sichon were not settled by those Jews and so in the sabbatical year have to bring the tithe given to the poor.
The second answer of RT leaves all the land of Sichon as one and considered it to have been settled by עולי בבל. So it is חייב in ביעור בשביעית. And Amon and Moav that are obligated in tithes to the poor are areas never owned by Sichon. At this point I don't have any idea what he would say about an area conquered by עולי מצרים. Off hand it makes sense to say he would go like the Rash Rabbainu Shimshon.
This second answer of Rabbainu Tam widens the borders of Israel considerably.
Rashi holds Sichon and Og is synonymous with Amon and Moav and are obligated in Truma and maasar by rabbinical decree. As for the Mishna in Sheviit one can say that is like the person that said קדושה ראשונה קידשה לעתיד לבא. [Rav Shach].
That leaves us with the Rambam. He holds עבר הירדן which is עולי מצרים is not holy, because it was conquered by Jews coming out of Egypt alone. Not Jews coming out of Bavel. And yet it is not worked in the sabatical year and it is חייב in ביעור.