Re: Hooking up the heater to a second radiator?
An auxilliary radiator is a good idea. I had one on my 93 dodge/cummins for the transmission. It had an electric fan that came on automatically when needed. Usually, when pulling loads.
I also like the idea of taking advantage of the heat if possible....but how often would that be. It is relatively simple to hook up a auxilliary radiator. It is a different story to build something to be able to use the heat "when needed". A plenum with cable operated doors could be built to make it work but I come from the KISS way of thinking and I live in Texas where heat is plentiful. It is not wanted except for about two weeks out of the year. Therefore the benefit is minimal to non-exisistant compared to the time needed to design and build the contraption needed to make it work. Do you ever overheat in the winter when you could utilize the heat inside the bus?
K.I.S.S. - keep it simple stupid
Did you address the other reasons for possible overheating yet?