Coming in from the outside is definitely an option, but it's going to put a hole for possible water ingress, into your roof. If you have the same style headliner as I do, removing one or two of the teak slats will allow you to drop down the white panelling.
Be careful when removing the panel (or panels) as they are quite thin and can be brittle after so many years and finding a replacement is a chore.
This will expose the wire chases and ceiling space. The "attic," so to speak.
You should then have sufficient space to mount a backer board or two using liquid nails or glassing them in or similar. You could then mount directly to those or drop the carriage bolts through from the outside while still keeping a good seal and keeping the roof rigid. If you don't want to drill from the outside, build a frame and then mount a sheet of plywood to it so that there is a hollow space underneath it.
Now, get a thin steel plate (another bit of plywood will do if you don't expect to ever mount a larger TV) about 2-3" wider and longer than your mounting brackets. Drill through the plate and plywood and mount your bolts so that the bolt hangs through the bottom, ready to accept your bracket and nuts.
You may also wish to drill a larger hole for the wires coming from your TV, to make use of the wire chase space under the headliner panels.
Last step would be to carefully drill holes in the headliner and fit everything together.
Hope this helps!