i have panels that can be removed to get to the tanks in front and removable panels in the rear to get to hydraulics.
i wouldnt count on the underbelly for insulation. it does protect from road debre. my tanks have some fiberglass on them; the underbelly protects that.
when i had frame cracks, i could see them from the side. didn't have to remove underbelly to see them. they did have to cut parts of it out to make the repairs. the way my underbelly is installed, the section above the axles is bolted from the top.