All that seems normal. Battery fluid levels get neglected all the time. When charging a depleted battery bubbling noises are common. The battery is not boiling, it is producing hydrogen and oxygen because of the charge current. I have a bank of 12 batteries at our off grid cabin and they can make quite a racket when the sun shines brightly and they are partially depleted. It sounds like the converter / charger cooling fan was running
I could hear the converter working
That too is normal if the battery was in a partial or fully depleted state.
A good charger, in proper working order, and properly adjusted if it has adjustments, will taper off the charge as the battery(s) get full. If we assume the TD in question has a converter like a PD4045 everything should be fine as far as charging parameters.
As to the question of what is needed to satisfy the DMV, it seems to me that the ubiquitous battery box we see sitting on the tongues of a great many small travel trailers is all that is needed. If that is what was planned all along, I'd go that way. If you want the battery inside then there is the question of how to vent the fumes produced while charging. A genuinely sealed box is a no-no. No where for those gases to go. The idea behind the rules will be to prevent accidental catastrophic shorts across the terminals and to dissipate any hydrogen and oxygen before an explosion can occur.
Depending on the DMV office useful info might be obtained from asking Sometimes that can be an exercise in futility and exasperation.
FYI, if anyone ever discovers the fluid level in the battery low enough to expose the plate tops that is generally bad for the battery. Don't fill the cells completely before charging. The electrolyte is a depleted battery expands as the battery is recharged. Just cover the tops and put the battery on charge. When fully recharged, top the water and charge some more. Again a good charger will have three stages of charge and will "sort out" what level the battery requires.