Smoggy

With double D rings one passes through the other if the rings are on each strap (mine is like that - Seago), if both on one strap the loose strap end would pass through the first ring, around the second ring and back through the first (been used on crash helmets for years). Check dates on activators and check for green dot on hammar avtivators, inflate overnight to leak check but use a pump not your breath as that fills the bladder full of moisture and rots the gas bottle. If in doubt have them properly serviced before relying on them.

Diesel heaters are not the only domestic use of fuel, just running the engine charges the batteries for light and heats water, that's domestic usage.

Plannar seem to be getting a good name and much cheaper than eber/webasto, look for owen cox on the ybw forums as he deals with them.

Did you buy a boat to be within 24 hours of a shore power hookup or to go boating? I'd go for the option that means you can spend a couple of days without having to run or be within hookup distance, you've paid more than the cost of a good fridge for the boat so why scrimp now and be reliant on inverters and big battery banks. Forget the 3 way options as on 12v they are absolute battery killers, 12/24v compressor fridge every time for me.