OP, give yourself credit - you are understanding the AC to DC conversion thing and that you need the current * voltage to provide the watts! Also the reality that while electric heat is efficient, it is not practical in a small portable battery system.
I kind of went on a similar path when I first started using our camper. After installing solar and upgrading battery capacity I figured I should use it to operate a 12V heater to take the edge off at times. But I quickly abandoned it and tossed it into the bottom of the camper gadget pile. So recognize that even a 12V heater that eliminates the need for an inverter at all will still not be great if you are using the stock 12V plug in the camper. In my camper I have another 12V plug that I wired in using some relatively large wire (8 gauge I think?) and it makes a huge difference on how that little heater performs. It can run when plugged into the stock 12V plug, although it barely produces heat. When I run it plugged into my fat-wired 12V plug it puts out pretty good heat. But it isn't warming a room up. Of course it is only a 150 watt heater/DEFROSTER and defrosting a windshield sounds realistic.
https://www.macys.com/shop/product/wagan-12-volt-car-heater-defroster?ID=10421642&pla_country=US&CAGPSPN=pla&cm_mmc=Connexity_PLA-_-n-_-n-_-n&m_sc=sem&m_sb=connexity&m_tp=PLA&CAWELAID=120156340009470470&m_ac=Connexity_pla&m_cn=n&cnxclid=16558304238181453857810070302008005