Are you sure you cant add something like this to your trailer that would mount on top of the tongue frame and mount between the Propane tank and the trailer wall..
photo from google images
You will need about 14-inches space. This will extend out over both sides of the tongue and if you just install two batteries in it will also give some tool storage space..
This diamondplate storage box is great for four or two GC2 6v Batteries and will give you around 12VDC @400AH of battery capacity if you install two groups or just 12VDc @200AH if you just install two GC2 6V batteies.
This also would be a great spot to lock-up your GC2 batteries for those off the power grid camping trips.
I can imagine space inside your trailer will be premium...
Be glad to share my info via emails if you might want to go this way... PM me...