We have a Vax carpet shampoo machine that we use once a year on the carpet. I think that would equate to the Dr one that you can hire. (We need the Vax in the house too, having a dog.)
We have a “bothy blanket” on the bench seats which keeps them clean, they get the washing machine rinse cycle only (as recommended by the Hawick weaving mill) as & when required.
I've got a couple of Vax carpet/upholstery cleaners (big upright and a spot cleaner), and used a variety of detergents brands without distinguishing between carpet or upholstery. Once the cleaner is set up, I'll often clean the carpet then spot clean some of the upholstery too, all with same detergent.
As to the caravan, I use throws over the seating (simply washed at the end of the season and fresh for the next year), and even with a dog who clambers all over the seats, never needed to clean the pale green upholstery yet - in 6 years of ownership! The light brown carpets on the other hand, that's a once a year task at least!