There are various manufacturers of rather expensive bespoke fitted caravan bed linen but be careful what you buy and make sure that if you have to return it for any reason you don't end up out of pocket like we were.
Forum rules quite rightly prevent me from naming names but when we ordered our n/s fixed bed caravan from a dealer at the NEC some years back a manufacturer and supplier of fitted bed linen had a stall at the NEC exhibition so we decided to buy a cut off duvet, a fitted sheet and a duvet cover, also with the cut off while we were there.
Unfortunately the duvet cover was substandard, the material 'bobbled' after just one wash, the duvet cover was really terrible and the sheet wasn't much better.
We contacted the supplier and returned the goods and were told that the material was part of a 'bad batch' and the suppler simply sent the same substandard items back to us.
We'd shelled out for shoddy goods and also for postage when we returned the goods, so to say that we were dissatisfied with the service would be an understatement to say the least!
Unfortunately I was quite ill at the time and not really up to fighting for our money back so we lost out big time.
Apart from that, it can sometimes be a bit of a struggle to put the cut off duvet into the cover and it's difficult to achieve the same look as that on beautifully presented beds on display caravans unless you want to spend a good part of your caravan holiday faffing around with bed making.
We binned the shoddy duvet cover and sheet and now use department store bought ones, we simply tuck the left over part underneath the mattress and at least the department store linen is of high quality with a no quibble returns policy, not that we've had to return any.
It's your choice but you don't really need made to measure bed linen and if tucking the duvet cover under really bothers you just turn the cover inside out, mark the excess with some chalk and get somebody with a sewing machine to run a line of stitches across with some edging tape and cut off the extra corner.