I have started collecting material to add the 2nd optional Group 27 battery to my 2004 Chevy Express 3500 Roadtrek Popular 190 this spring. This was an option through 2009.
In 2010 Roadtrek started putting two taller batteries in front of the rear wheel in a single compartment. This can be retrofitted into an older van as per Booster's post here:
As of 2020, the larger battery compartment door can still be found at Mobilife but they don't have any tray parts.
What I am trying to do is very simple because the storage compartment in the 190 is already big enough to accept a Group 27 battery and you can buy the optional tray from Mobilife (this is new old Roadtrek stock, I'm not sure how many they have).
The installation is supposed to be as simple as drilling some holes in the side of the compartment and bolting in the tray. You could save some money and make your own tray if you want to.
The reason I am staying with Group 27 batteries instead of installing the more desirable GC2 (Golf Cart 2) batteries is that there are quite a variety of deep cycle Group 27 batteries that could meet my requirements without giving up too many amp hours.
This job will take a few months to complete because my van is in storage so I'll split the post up a bit. This will also help with managing the photos.