Northern Lite specifically recommends 1' of plywood with a rubber mat directly under the TC..at least that was the response I received for our 2016 NL dry bath and 2016 F350.
More importantly I understand from other research that Northern Lite does not recommend using any of of the foam board insulation...and this may also be true for Big Foot TCs. Something to do with the fiberglass construction and that much of the weight is concentrated at the bottom of the TC walls instead of equally spread out across the entire bottom...apparently too much for using the foam board especially considering the dynamics involved in taking turns etc. which obviously increases downward pressure at along the edges of the bottom.
Keith
