  The neutron structure function, $F_{2n}$, has historically been extracted
from measurements of the deuteron structure function, but our understanding of
the nuclear effects on the bound proton and neutron limits the extraction of
$F_{2n}$. The MARATHON collaboration recently extracted $F_{2n}$ from the
comparison of $^3$H and $^3$He targets, where the nuclear effects are larger
but nearly identical, yielding a precise extraction of $F_{2n}/F_{2p}$. This
precise extraction can then be compared to deuteron extractions, providing
important constraints on the nuclear effects in the deuteron. To ensure that
this comparison is not biased by the specific model of nuclear effects used by
MARATHON, we examine a range of models of the nuclear effects to obtain a more
conservative, but more model-independent, extraction of $F_{2n}/F_{2p}$ for
comparison with deuteron extractions. Even with the more conservative approach,
the comparison suggests the need for significant off-shell corrections or other
nuclear effects, beyond those include in most calculations, even for the
weakly-bound deuteron.
