I have it next to the galley sink which is in the center of the boat.
If you can find the space, permanently mounting with some ducting seems good.
It's not quiet, we turn it off when at the boat and put away when
sailing. I haven't noticed it generate any heat.
There's no reason to have a/c here.
Having grown up in southern Ontario, the Humidity in Seattle definitely seems lower. We don't get the condensation on everything in the cold months.
The dh5 is a pretty low cost thing to at least try out. The mermaid was smaller so might fit out of the way. Fitting into the ducting for the heating system sounds good.
In another thread someone mentioned that ventilation is counter to running the dehumidifier. However we do have two solar vents and four passive dorades. I haven't tried closing the boat up to see an improvement.
With your size you should consider hydronic over forced air for heat.
Someone else noted in an older thread that ventilation