On the 2004 Roadtrek 190P, the house battery is in a small compartment on the passenger side behind the rear wheel. It is only big enough for one battery. The battery is on a slide out tray.
I put the second battery in the small compartment on the passenger side in front of the rear wheel. I did not use a slide out tray.
Home & Park sells a complete kit for this. It has a second tray that would install in the compartment in front of the passenger side rear wheel well, two cables ( Positive and Negative) with battery ends, vents for the compartment modification, wire protectors for the entry in the compartment, bolts, nuts and washers and battery hold down.
I didn't purchase the kit. I 'saved' $300 but, if I had to do it again, I'd probably buy the kit. If I had done the entire job myself I would have saved $500.
I paid a company to make and install the cables which took them 2 hours. It took me another two hours to fix what they did. The cables were getting kinked each time the slide out tray was pushed back in.
The two batteries are connected in parallel. I have 100amp fuses on each end of the positive cable that connects the two batteries. No fuse on the negative cable.