TonyTiger,
Thanks for the advice.
I contacted DOW Chemical regarding the product (Great Stuff Ployurethane Foam) and was told that the product doesn't contain formaldehyde and would be safe for use in an enclosed living space (rv). However, the product and its fumes are highly flammable during dispensing.
In addition, some discussion was had regarding the application of the 'expanding' foam and the possibility of the foam bowing the panels outward. They suggested applying weight to the upper panel and upward presure to the bottom panel and apply the product slowly to allow it to expand properly in the direction of least resistance.
That said; I'm seriously considering 'Plan B'. Measure, Cut and Place 1/4" Plywood bottom panels (floors) into the cabnets (on top of the existing). This may be a better approach.