You do not want a battery in any "upper" compartment for several reasons.
They're heavy, for one, and keeping the center of gravity low is good. Also, if you mount the battery up high somewhere, you have to have a place for the battery that will support a heavy battery and high live loads from traveling over the road. The original battery boxes are generally supported by the subfloor or the tongue of the trailer (depending on the model and year.)
Another reason to keep the battery as low as possible is that liquid flows downhill for the most part, so the lower the battery is mounted, the less there is for corrosive electrolyte to contact in the event of a mishap.
Are you describing the "drop down" battery door that fits into the lower curve of the trailer back near the bathroom? I know some '70s Overlanders have those (I've seen one, but don't remember what year the trailer was.)
As far as the vendor for the battery, I've done OK with O'Reilly's house brand of Group24 battery in my Argosy. I've heard good things about Costco's Group24 and Group27 batteries, but I have no experience with them. There are WAY more battery brands than there are battery factories, so the trick is to find out who made the battery rather than what brand is on it.
Zero Gravitas — 2017 Flying Cloud 26U | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill