We lived aboard a GRP Broads cruiser and we were always warm, no condensation issues and I fitted a Glembring drip feed diesel heater which meant having a chimney which was not a problem to fit.
As it was built circa 1980 it was built like a tank. Back in those days GRP was relatively new material for boats and you can imagine the conversation, " well how thick shall we make it? I dunno, 1/2 inch? Nah make it 3/4, OK"
To put a through hull fitting in I had to go through from both sides with a hole saw. The boat was 40'x12'
Phil