Norm,
The battery configuration that you use has several variables, mostly depending upon the way that the coach is used and your personal preference. First thing to consider is, do you ever let the cranking batteries excessively discharge. If the answer to this is no, you do not want deep cycle batteries.
The next issue is price. If you can get a good quality 4D battery for about the same price as a group 31 battery, then why not use the 4D? However if they are substantially higher in price than a group 31 battery, I would op for a pair of group 31 batteries. I would not recommend using four regular automotive batteries. The group 31 batteries are designed to start large diesel engines, they give excellent service in that application, and that is the cranking battery that is used in 90% of Beaver coaches.
Gerald