The manual I have shows that fuse #5 controls the instruments but also the fuel pump, main power and starter relays, so IMO if the bike is running it's not the fuse causing the issue with the speedo. I have no idea if the possible internal corrosion from our damp NW weather could cause issues or if said issues can be corrected. At the very least make sure the connection is tight and corrosion free. Hell, take it off, take the cover off (careful it's brittle plastic) and soak it in a bag of rice, what do you have to lose? It did work prior to parking the bike, right?
If possible try hooking another horn up to check if the circuit is good. The horn not working might also be the horn itself from the bike sitting for so long. SOMETIMES they can be fixed by loosening the adjustment screw on the back (if there is one) and applying power to loosen the diaphragm. Other times spraying the hell out of the internals with WD-40 and smacking it on a hard surface may break it loose.
Is it just me, or does it seem like a lot of Rockets end up sitting waiting for the owners to get back to them. Off the top of my head there's Miami Dave's, Sojerguy's and Bishop's. Mine also spent the last few years getting minimal use while I screwed around with wiring issues. Yet we don't part with them.