Surely, you can use a USB backup battery to run your dashcam 24/7, no? And have the USB backup battery charge when you drive? How much power do these dash cams draw?
Blackvue provides a fairly handy table. The single camera setups draw around 2-3W depending on your connections, and dual camera setups draw 3.5-5.4W.
Dashcam Comparison Table - BlackVue Dash Cameras
Your typical USB TSA legal 100Wh battery bank provides 18 to 25 hours of battery life per camera. Note the largest pack (B-124X) Blackvue supplies is only 76Wh and costs a whooping $370, although they do offer $300 85Wh expansion packs (B-124E).
If you get third party generic power banks, note you have to find one that allows passthrough charging, which many don't. Also note the dashcams usually don't run on USB, they run on 12V, so you must get one that has 12V output if you want to use most dashcams (unless you shoehorn a security camera for the application like I do). Also most power banks have a charging time of around 8 hours, so your typical drive will not be enough to charge it.
For my personal application it simply won't work, as I will park well over that amount of time. My Wyzecams are actually more power efficient: 4 cameras + router only draw 7W. There are battery powered outdoor cams that draw less power, but note those usually use a separate motion sensor to save power and may actually miss the event that triggered it (instead records a few seconds after). The ones that have no delay or even pre-record, will likely draw around the same (2W per camera), given they have to have the camera on all the time (and for pre-recording needs to at minimum buffer to volatile memory). I'm guessing the OP wants something more like the battery powered outdoor cams, but in dashcam format.
The USB battery packs are not cost effective vs even a 12V LiFePO4 battery (lead acid ones are even cheaper if you can deal with the size and weigh). I paid $300 for two 30Ah units, which using Blackvue's 12.8V nominal accounting works out to 768 Wh of capacity, or $0.39/Wh. That means 100Wh banks would have to cost under $40 to be the same cost, and if you look around, that usually gets you generic ones that typically don't actually have 100Wh in battery cells inside, plus there is more conversion loss from stepping up from 3.7V nominal to 5V (probably 80% efficient for the good ones) vs stepping down from 12V (the 4 port module I use claims 95% efficiency).