I have noticed that many TT's use a battery tray that is attached under the frame .
Wouldn't it be a stronger tray if it was on top of the frame ?
Personally, if I was adding 2 grp 31's or a "solo beast" like the 8A8D , I would build one on top and use the OEM tray as a support by running vertical supports.
I think people shy away from the solo battery idea due to the weight and the strain on your back to lift it . Its very difficult to lift using your leg muscles at such an angle behind the propane tanks.
I carry extra batteries of the same size,type and age , in the trucks bed and wire them up when boondocking. Most of the time a smaller battery in the TT tray can do the job without worrying about the weight of the battery or added trailer tongue weight.