I agree with everything @Nat6999 said about puriclean and water, mattress toppers and propane.
We mostly stay at Caravan Club sites and do drink the water from the taps there, but not the water that's been through our caravan water system. Our caravan has a fixed water tank like a motorhome and like to have a serviced pitch so it's easy to fill the water.
We use Puriclean twice a year, once in the spring before the season starts for us and once at the end of the summer/early autumn when we put it away for the winter.
People will all use this kind of layout differently but yours sounds similar to ours, although ours is probably smaller. We don't bother turning the bed into a table/seating area, it is like a fixed bed, although the mattress topper/duvet can be folded back for access to the under bed storage. We sit on the bed with a tray to eat, or sit outside if it's warm enough.
I'm a bit of a 'princess and the pea' when it comes to beds. We have a thin memory foam mattress topper in a cotton cover we got from Putnams.
On top of that we have a lovely wool mattress topper we got from Devon Duvets, with a wool fleece blanket on top, both of which are inside a single duvet cover that is our bottom sheet. We got the single duvet cover from IKEA as they have zip fastenings on some and the single fits our double mattress topper perfectly. Both Putnams and Devon Duvets will make topper to your exact size requirements if you measure the bed.
We have 2 silk filled summer weight Duvets from Soak and Sleep. We really like them as they are light and thin but warm and 2 can join together to make one for colder weather. We also have a couple of wool blankets that we can pull over the duvet if it gets cold in the night (for me, DH doesn't need them).
I'm sure using it as a hobby room would be fine, but consider the tyres if you are not going to move it for a long time. I would also have it serviced if you are going to tow it, unless it's been done recently. And if you're going to use it as a hobby room, with the gas it would still need a habitation service.