I have been living on my wide beam 60 x 12.6 for around 8 months now, basic sail away new in Dec 2019 - I installed a 6.4 kW Multi fuel stove. Stanley Oisin. We have had snow and frosty mornings in Ireland - Our doors both front and rear are not exceptionally air tight, and all our vents are open fully. We get around 20 C with the stove fired up all day - this is very comfortable.
I was against the multi-fuel and still am. My wife twisted my arm on this but is now more open to changing to a diesel burner like Refleks - I have my heart set on an Effel diesel heater with scope for radiators - this model is around 4.7 kW on the stove alone - another 8kW is available with the radiator coil and radiators - dependant on the number of radiators - I'm not too sure how the manufacturer determines this output, but I trust the information so far.
I am also installing a diesel powered air heater - As Alan has mentioned, this is for "instant" heating on unexpectedly cold mornings in warmer seasons...