For me I think the answer has to be
a kettle and brew kit
There are of course alternatives that will do the job but the thought of not being able to put the kettle on for a cup of tea, is just unthinkable, too unspeakable for words.... and might lead to the end of my caravanning.
Motor movers, televisions, duvets, fancy lights and electronic devices,, ehu, central heating of one sort or another and all the plastic knick knacks that a lot of folks think essential, maybe are nice but in my opinion all pale into insignificance compared with the kettle and brew kit!
As always regards to all, and of course,
Happy Caravanning
paws