I gather that some people are happy with their propane heaters. Just make sure it is vented. My only experience with propane heat was in someone’s
cabin where it was unvented and the moisture produced by the burning propane made it very uncomfortable.
Your boat is the same size as ours and I believe your temperatures are somewhat similar though a bit warmer. Hovering around 8- 10° C most nights at the moment. The Dickinson solid fuel bulkhead heater was not able to heat up even one of our boat cabins with the doors closed, no matter how dangerously red hot we got it. We
sold it to a 20 foot boat and it wasn’t hot enough for him either. The Grizzly would be the same in my opinion so unless you’re really only looking for a boost of a few degrees (and don’t mind constantly feeding the thing) I’m pretty sure those two would not work for you.
What has made the most difference for us at the
dock is a dehumidifier to take out the damp. For the wattage involved it has a lot more effect than the electric heaters do.