I find riding a trainer pretty boring and try to avoid it. I am not saying others will or should feel the same way, but it is something to consider. It is hard to stay motivated if it is boring. That said I have found that I can stay in pretty good touring shape even when I am not riding much. Both running and indoor rowing seem to keep me in pretty good shape for touring.
Before my spring tour I had only about 200 miles in for the year. I had been running and trail running and had run a half marathon a few weeks before the tour. I was able to average 80 miles per day for the 10 days of riding and even managed a 142 mile day at the end of the tour. I did all this in reasonable comfort with a base of about 200 miles.
Similarly a year or two ago when I started riding after a winter of indoor rowing I found myself to be in pretty good riding shape (for some reason I find indoor rowing less monotonous that riding a trainer). I am 58 years old and a mediocre athlete at best, so if it works for me it probably would for most folks.
My point is that maintaining general fitness is the main thing and to be in reasonable touring shape doesn't necessarily require a great deal of riding miles.
I'm not knocking the trainer if it works for you, merely suggesting that other options work well too.