Not that I have fully digested your intent & system...
But 30W is a relatively small power requirement - ie, well under normal >90W DC-AC inverters and DC-DC PC supplies.
For powering in a car etc, a DC-DC converter is probably best, especially when designed with low standby currents (as in uA = micro-Amps).
The exception might be when using more common AC devices - like PCs, phone chargers etc - without the need to get DC equivalents.
Since your system has a DC input, there is no point going DC-AC (inverter) and then AC-DC. Better going DC-DC.
A 30W output is 6A at 5V for USB which should a similar 6A input for a "linear" voltage regulator converter, or maybe 3A for a 12V-5V "inverting" converter. (12V@3A = 36W; ie, 30W out & 6W = 20% inefficiency.
BTW - "inverter" sometimes means DC-AC converter, but since DC-AC-DC converters use "inverters", are they also inverters despite their DC output? The true jargon is probably SMPS = Switched-Mode Power Supply which is the "DC switching" used for DC-DC via an intermediate AC transform(no pun)ation.
I'm saying DC-DC, not a DC-AC "inverter".
If you load is happy with a normal battery and car "+12V DC" supply - which means typically 11.5V to 14.5V at the battery excluding occasional dips down to perhaps 10V or 8V due to cranking, or headlight switching etc, the you can just connect it to the battery (via a fuse etc).
Though usually isolation, transient protection etc are required.....