I camped in the back of a 6 horse trailer as a groom with my rider for 2 seasons. I loved it. Super easy and we had loads of space - I preferred it to sleeping in a tack room as those make me claustrophobic. We hosed out the horse space (never used shavings) and removed the dividers. We usually just slept in the trailer lot.
We had a big cooler that I filled with ice on Friday. All food was kept in there and stayed cold until Sunday. We kept the cooler inside the trailer with us. I have this grill and cooked all of our meals on it. I bought frozen burgers for Friday night with green beans or asparagus grilled in foil. The burgers thawed in the cooler until I cooked them. Then we did marinated steaks the next night or the competitors party. I made paninis for lunch (usually just lunch meat, chipotle sauce, sweet peppers, spinach, cheese). For breakfast, we did bars or I got precooked eggs in the freezer section and warmed them up in foil along with precooked bacon, cheese and bread for breakfast sandwiches. I always had antibacterial hand wipes around for hand washing. We have an electric kettle that we plugged in in the barn for instant coffee or tea in the mornings. We ate on one of these tables which I loved with camp chairs.
We slept on a tall queen blow up mattress. Blew it up in the barn or at a friend’s camp spot and carried it to the trailer if there weren’t plugs nearby. You could easily get an adapter to blow it up with a plug in the truck with it running.
To shower, I hosed myself off in the wash rack in my sports bra and underwear late at night or filled bucket and dumped it on me with a solo cup behind the trailer. I got one of the solar shower bags, but the water pressure was so low that it took to long and I went back to the bucket method. I am too lazy to heat it, so I just left it out all day in the sun to take the edge off. Brushed teeth and washed face with water bottles. We always purchased gallon jugs to drink out of so we used those - at least 5 per event.
We lit the whole back of our trailer with two strands of battery operated twinkle lights. We could easily read with just these and never used the trailer lights. Phones were charged in the barn.
We never used a fan. I am sure you could easily get a battery operated one. This one on Amazon looks interesting and cheap.
Once you have a process it is super easy. Our owners usually cleaned the trailer out for us (about 20 minutes) while we dealt with the horses and then my rider did the set up in about 15 minutes after she was done riding.