On the OA front, it only looks backward 2 years from the present. Based on that, I would try to catalog camping within the last two years in some “detail” (dates, nights, scouts & adults by event). Then, you could pursue @DonovanMcNeil’s approach of “smearing” the information as needed to get it in the logs.
Personally, I’m enough of a twitchy engineer that I would probably see what the text/CSV export file from TWH looks like, then try reformatting/pre-processing that in Excel so that I can more easily generate the input data to add to Scoutbook manually. For example, create an “event” worksheet that more or less matches the layout of the campout event interface. Then, figure out a way to scrape information related to that event (e.g. event name) from the export file. That information (start & end dates, # days, # nights, list of scouts & adults, other misc info) would get compiled in the “event” worksheet, so I could more easily transfer it. Repeat ad nauseum for other events that have logs implemented (currently hiking & service).
I would work backwards in time creating the events and input everything as it existed. But as I said, I’m a twitchy engineer. That doesn’t necessarily make me the best role model in cases like this. :^)