I agree with Dan, there should be another piece to hold the battery in place and do when they come from the factory, but are lost quite often by the mechanic or owner.
I also have a 2002 Roadtrek Popular 200 that has the same battery arrangement. I purchased the RV used and the dealer installed a new engine and house battery prior to me purchasing the RV. I also discovered the battery hanging out of the battery tray, held only by the negative battery cable.
I solved the problem by using salvaged metal and rivets to make a slide out extension to the existing tray. I used steel, but it would be easier to purchase and use aluminum sheet metal for the extension. The extension is long enough to allow a size 31 AGM battery and slide 3 inches inside the existing battery box. It has a bottom, two sides, and a end, basically it is half of a rectangular box. I manufactured a L clamp that attaches to the mounting bolt for the generator hanger very near the end of the new box to hold the extension in place. Looks great and works great.
I realize that this is a 1 year old post, but I'm sure that owners are still having a problem with this, so I thought I would give those my fix.