Welcome Dolly from myself as well.
As to your cold bedroom at the back of the ‘van, you’ve realised the issue. If the room is the full width of the ‘van you have 3 external walls and the floor and room all helping that room react to outside temps.
Your suggested solutions would improve things in relation to one of the five external surfaces(floor) but the others will still be the same.
Have you tried rugs, or carpet runners as a temporary help for the floor before you spend larger amounts on additional insulation be it underfloor, or above floor (better carpeting &underlay). All of which would improve the situation a bit but it will still be a colder room than the rest.
We have an oil filled rad as an extra heat source for when needed if the CH is off etc. but also have electric blankets available.
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