Why are you against going the 12VDC>110AC>48DC route? an inverter that can handle 300 watts can be had for around $35 - that’s 25 (rated 30A max) amps at 12v and 6.25A@48v (I know efficency comes into play here). They also recomend hard wiring if you are going to use more then 12 amps. I got this one over 2 years ago so they are most likely cheaper and more powerful by now.
Going from 110 av to 48 dc should be a piece of cake.
The unit is fairly small (about 1.75x3x5in) and has a automatic cooling fan.
If you want to go the motorcycle batts, you might be able to hook up 4 in parall while not in use so they can charge then have a complex switch that would put then is series while in use - most likely more costly then the inverter route.