If you can manage the size, T48 is a way better choice. Outdoors, yes, Tubas work. But, their true potential is unlocked indoors. Titans, on the other hand, bring a huge sensitivity benefit to the table. When there's no boundary loading to take advantage of, you're going to really give up a ton of output potential going with a Tuba. Not to mention power requirements will be more demanding on a mobile, battery powered rig with Tuba's. A 2x12 or single 15 loaded t48 at 30" width would be your sweet spot. If you're using automotive amplifiers, the 2x12 option would allow you to wire for a 4Ω load.
Either OT8 or OT12 will suite well. You'll only need one unless you need/want to have them aiming different directions for additional coverage, perhaps on either side of the trailer. The OT8 will cost a little more.
There are 12v DSP options available. I think MiniDSP has everything you'd need, including a peak limiter for the sub (fairly important).