I have bought a few cheap powerbanks and have had great luck with them. The last one I purchased was
this one (for $34.99 at the time) from Amazon in February of 2018 and it is still going strong which is especially noteworthy as I use it daily for charging the GPS/Camera and other devices. I can't remember the last time I plugged my GPS/Camera into the wall, but I plug this into the wall one or two times a week to charge up. When I'm tripping it goes in it's own ziploc bag which goes inside a dry bag alongside my extra clothing.
I have heard that powerbanks will go bad from lack of use. If you don't use it everyday, make a point to use it and then fully charge it every couple of months. I do this with a couple others that I had before I bought the RAV and they still work.
Airplane mode is key! This keeps it from trying to contact a tower which drains the battery very quickly when there is no tower to answer. I spent 21 days camping in Idaho a couple years ago and kept my GPS/Camera on and ready to take pictures the entire time with about 34k mah of charge coming from 3 separate powerbanks. I made exactly 3 high-elevation phone calls on that trip but I took pictures every day and even read a couple books. You want to make sure it is not doing anything so turn off Wifi, bluetooth, hotspot and everything else that you can. I also close all apps (camera, maps, Kindle) every time I am done using it and I believe this helps as well. Shutting the GPS/Camera off altogether holds the battery longer, but then you have to wait for it to boot up before you can take pictures. Turning itself on normally takes a couple % of the battery as well so I have found it best to leave it on, even at night.
For someone who has received exactly 0 photography lessons in his life, the quality of photos I have taken with my GPS/Cameras amazes me. Currently my GPS/Camera is a Samsung Galaxy S10, but I've had an S3, S4, S5 and S7 as well.
Edit: For context, my regular, everyday use of my GPS/Camera includes phone calls, text messages, surfing the web and broadcasting a mobile wifi hotspot for my laptop. In this function I have to charge it at least once daily and the 26,800mah power bank I use will keep it charged for 2 or 3 days at most.