Thanks Bob, Bruce & Rachel.
Yep, I was surprised to find the 1/2" subfloor. Mine is a 1981 Excella. Since most of the floor (except for the 4x8 sheet in the back and a small soft spot up front I need to patch) is still holding up strong, I thought I'd go with what was there instead of needing to sand etc to get the pieces flush.
I hear you Bob about the Reflectix not doing much for insulation the way/place I'm installing it (with no dead air space other than the bubbles themselves). With the belly pan on, I can't figure out a way to install foam board without just laying it on top of the belly pan--that would be too much dead space but at least it wouldn't soak up water like the fiberglass.
I'll go back over to Lowe's Hardware tomorrow morning (I'm there like every day
to see if they have an epoxy with lower VOCs.