She is plywood planked. I would recommend lifting ALL the floorboards to see what everything looks like below them, that is where most problems will be found. Check that all the plywood is solid. Also check out all the ribs, typical location for cracked ribs is aft, at the turn of the bilge. Look under the "cleat" (strip of wood) that supports the outer edge of the floorboards.
Another typical problem area is up forward where the keel meets the stem. Many 'less knowing' people would leave the boats outside nose down on the trailer and rainwater would collect and pool up forward causing problems.
You should also inspect how she is sitting on the trailer. Is the bottom hogged at all? Is the bottom mis-shapened at all where it sits in the trailer (rollers or bunks?). Good long bunks are better than rollers. If there are bunks supporting the back third of the boat (usually about half way out from the keel), are the ribs in good shape above the bunks( this can be s place to see them cracked.
Also check where the planking meets the transom.
I would suggest checking out the Thompson boat owners web site.
http://www.thompsondockside.com/ There is information available there specific to Thompsons that can't be answered here.
Good luck checking her out.
