installing second battery in 2004 roadtrek

Senf, I don't know why yours would be any different.

This from the 2008 C 190 owners manual, which is the same as the 2004.
All years Chevy 190's till late 2010 where this way.

"Behind the acces door behind the rear wheel the following is found:
1. Slide out tray with auxiliary battery"

"Behind the lockable access door in front of the rear wheel, (see Picture B-P47), the following is found:"
1. Storage area or second auxiliary battery, (when installed), (190 models only)."

Roadtrek 2008 Chevy 190 Owners Manual

Interesting! My two batteries are reversed to yours. The optional battery being behind the rear wheel. Both batteries are seated in metal trays. The metal trays are attached to slides which are also attached to the sides of the compartment. The battery weight is distributed across the metal tray and then onto the slides and the sides of the compartment. If the optional battery is installed at delivery, I very much doubt that any reinforcement would be needed, however, there would be something to enable the battery to be held rigidly in place - such as a tray.

Roadtrek Service will no doubt have an answer.

Connecting two 12 volt batteries together requires parallel connections - positive to positive, negative to negative. Use the same cable thickness or gauge as the existing battery.

If it was designed as an option to hold a battery I assume it would work. Maybe a call to the dealer would help.

Batteries should always be of the same age and brand. Replace the original one at the same time you add the new.
If you go with AGM batteries, the compartment will not have to be vented. In fact, if there is a good space close to the current battery the new battery can be inside the rig.