Same with our 30DS. I haven't investigated it yet, but I got to thinking while I was installing our CB. If you look under the driver side floor, I think I remember there just being a sheet of steel instead of some plywood. So I wonder if that has something to do with it. Granted, the overall heater output isn't much. Also wonder if we could tape off the outside air intake so its restricted to take in the cabin air. I'm curious if anyone knows more.
I just ended up driving with socks on, generator running, and with a space heater up front. Made it tolerable for our trip over xmas.