Not a 3.5t but I've slept in my Equitrek a few times, thankfully I have fairly clean travellers, so I power wash it out beforehand and pop some shavings in for the journey. I have the partition over to the side anyway, I take cardboard and an old rug, these gets laid down on the floor, then I pop up a camp bed and use the back door for access. I taped something over the windows so it was dark at night.
Have successfully stayed on my own and with a small child at a games comp! It was so nice being able to stand up straight and not be in a tent.
At camp I swept the little amount of shavings to the back, put down a lightweight tarpaulin, added a pop up tent with a foam mattress and sleeping bag plus trakkie bottoms and sweater as it was cold weather. I also have a small portaloo in the back. There was space to keep my clothes and get dressed in the back.
F had a stable and hay and tack storage space. All meals were provided for me.
I would create a frame that could be folded up, and have a mattress made of foam so it can be rolled up and travel with that in the car, then put up across the living at the back. Easy to do if you have basic DIY skills or no someone who does and really simple and hassle free to use.
I stayed away at camp when I had my van conversion. I got a camp bed that unfolded like an umbrella, & a sleeping bag & set it up in in one side of the horse area. Tip: check that your lorry doors actually open from the inside. Mine had a lever on the back door which said 'pull to open' and it turned out in the middle of the night when I needed a wee that it wasn't connected to anything & didn't work. I managed to open the doors above the ramp & had to climb out over the ramp! :-0 Despite it being May, it was also very, very cold.