I agree with Mustangman that it’s unlikely that both grounds would fail at the same time, though it is possible. I would look for something that all these lights have in common. It could be the switch on the steering column or dashboard, a relay, or a wiring harness connector–maybe one near the battery got acid on it. The lightbub sockets can also cause this kind of probem, but would they all go bad together?
A wiring diagram would be most helpful, but you might need a little tutoring in how to read it. Basically, the lines in the diagram are the wires. find your non-functioning lights in the diagram, then follow the lines to wherever they go. If you come to fuses in the diagram, continue folowing the line past them: You already checked them. There might be something (relay?) that the lines from all the lights converge on. That is the item you want to check. The diagram should also show wiring harness connectors, which could cause your probem.
The symptoms are cassic indicators of bad / corroded / dirty connections or relay contacts. If the relay housing can be opened, the contacts can be cleaned using a sovent like MEK, or using emory paper. If the contacts are burned or pitted from electric arcs, you can try a fine file–like an old-school points file–, but they may be damaged beyond repair.