I tend to suffer exactly the same problem with my Dall Kirkham. Rarely any moisture when observing but if I check the scope during the day there is often condensation on the primary at this time of year.
My scope has three small cooling fans behind the primary. They pull air down the tube from the open/secondary end past the primary. They are very effective at clearing the moisture even though that isn’t their primary purpose. To combat any dust concerns I put a doubled up old piece of net curtain over the open end to act as a filter but to ensure good airflow.
I’ve recently started using packets of silica gel inside the tube when the scope isn’t in use. This seems to help, as long as they are regularly recharged in a warm oven and the tube is fairly well sealed.
It’s a really annoying problem that I’m still working my way through.