Spongy floor fix right at entry door?

[tundrwd]

I've got a spot - about 4-5" x 4" or so right in the middle of the entry door, immediately behind the metal threshold that is spongy.

This is the spot were most people would normally put their foot when entering the trailer. I haven't been able to get under it yet - I'll have to remove the step box to look under it to see what's going on, but I suspect it isn't properly supported under the step box, and since it's right on the edge of the flooring, has "given way".

Anyone else run into this - and what was your fix?

I've been thinking of taking out the step box, then laminating another piece of plywood under that entire area with some structural epoxy. I don't think there's enough room between the step box and the flooring to allow much more than a 3/8" or 1/2" thick piece of plywood - but not sure.