I’ve carried one for years. First was a lead acid and now a L-ion. The best use has been your buddy comes over and needs a jump start, they expect you to close up your camper and drive over to their spot. With a portable, you point to the storage bin and tell them to put it back when done. The old lead acid had a compressor and that was handy for bikes and toys. I now have a separate 12v compressor. When boondocking, I also use the portable to run a water pump for transferring extra water to my main tank.
The few times I’ve needed a jump start, I was able to use the Ford onboard/ built-in booster that uses the coach battery.
The newer lithium batteries are small, clean and hold a charge for a very long time- 6 mos to a year, maybe longer. They also usually come with usb and 12v outlet. The brands keep changing, check for online recommendations.