Since nobody else has asked:
Where are you going to MOUNT this second battery you now have ?
If there is space adjacent to the first one, I vote for connecting them in parallel and treat it as just a bigger battery.
The worst thing you can do for a conventional lead acid battery is to run it way down and then leave it sit that way for a long time.
It is nice but not really necessary to have the coach charge from the truck.
If you do that, you must be sure that the truck will not DIS-charge along with the coach batteries.
Armed with the advice your getting here, I kind of get the feeling that you should let a local service shop help you make the final decision about modifications.