Chinook RV

Our '94 Premier has the coach battery under the hood on the passenger side and the battery tray measures 5.5" x 13", which is very narrow.
Does anyone know what original battery fits this ?
Right now it appears to have a group 51 battery, which is pretty small. The starter battery is a bigger group 27.
What are others using in it's place ?
Thanks
Ron

The chassis battery is the one that is under the hood, and it does not need to be that big if all it does is crank the starter for the most part. If you have the one that have both the chassis and coach batteries under the hood, I have no idea, but I am sure someone else will chime in.