Bob and others: I installed two Interstate batteries, too, but spent some time looking for a single battery case. I found (and ordered) a battery case specifically for golf cart batteries from here -- http://http://www.allbatterysalesand...fm/4,576.html.
After receiving the battery case, I discovered that the battery case was about 3/8" too wide (
) to fit between the angle iron brackets for the battery bank. To resolve this problem, I took my trailer to a truck repair shop and had a welder cut-off one of the angle iron brackets and re-weld into place to accommodate for the wider footprint of the new battery case. Since the angle iron was only slightly moved, the distance is such that it still can hold a group 27 type 12V battery. See the attached pics (note: since taking the pics, I have upgraded to 4 AWG wiring; the pics show 8AWG wiring; also, I insert a piece of plywood between the two batteries to ensure that they do not move inside the battery case). I have not, however, securely attached the battery case to the A-frame with U-bolts; instead, have relied on the weight of the two batteries and battery straps to keep in place. I realize that it would be best to do so, but I have not had any problems with this configuration after traveling well over 8000 miles over the past three years.
Before going on a trip, I place the battery case on the A-frame. I then take nylon rope (rated for 120-150lbs); insert the rope through the inlets/brackets on top of the interstate batteries and carefully lift and place inside the battery case (note: I called Interstate Batteries to confirm that the inlets/brackets can be used to lift the batteries as opposed to just using them for tying-down the batteries); insert plywood between the batteries as a spacer; attach wiring to batteries; and finally, securely attach the cover to the case. If I'm transporting my TT to storage or to a repair shop (which is not too often
!), I just attach the good 'ole group 27 12V battery. When not traveling, I keep all my batteries charged using a BatteryMINDer Battery Charger / Maintainer with Desulphator - 12 Volt 2/4/8 Amp, Model# 12248.