Frost needs water
vapor to form. I think it is important to realize that this water
vapor can enter not only from leakage at the seal, but also from food
that is not in sealed containers. As an extreme example, if you put in a head
of lettuce that has ben washed, then I believe the moisture from the lettuce will greatly contribute to ice buildup.
I read an article some time ago, I think by John Neal, that suggested using desiccant cylinders by H2Out to keep the inside of the freezer
dry. We bought a couple of these on Amazon and have been happy with them. Just don't buy one too small, or you have to "recharge" it more often. We do this by heating
them in a heavy pot on the stove.
And as always, I appreciate Rich contributing his knowledge to this forum on the refrigeration
issues we cruisers are always dealing with.