The 4 each 6V deep cycle golf cart batteries (the T105 size that came with the coach, or the larger capacity T125 version) are MORE amp hour capacity than a pair of 12V lead acid batteries that would still fit under the hinge rod on the front cover of the Tour Master coach.
These 6V cart batteries in this configuration (series/parallel as shown in the Trojan user's guide at
http://www.trojanbattery.com/Tech-Suppo ... sh_003.pdf ) provide about 450 amp-hours of 12 volt power to run your inverter.
By contrast, the expensive and massive Group 31 12v starting battery for the diesel engine is only rated at 100 A-H each, so two of these would only be 200 A-H -- and therefore just under
half of the storage capacity of the four T125 batteries... The group 31 PC2150S by Odyssey costs about $350 each, some brands like Deka, Optima and Interstate are less money, but are also less capacity -- The Optima is 75 A-H for instance. These starting batteries
do have the ability to provide more short term current for the high demands of the diesel starting motor, around 900 to 1000 CCA, but are not the best type of battery to use for sourcing the inverter.
In short, you want to maximize the CCA for a starting battery, but the deep cycle use for inverters requires that you maximize the amp-hour capacity of the battery. It is like comparing a draft horse to a pony express mount -- different use environments entirely.
Hope this helps...
Chuck