Have redone the covers on a couple of trailers as well as some items in my home.
You can simple use the foam as your pattern. As others have suggested using large rolls of brown paper wrap to trace the foam out allowing for seam allowance works well.
How much material you will need will depend greatly on whether or not there is a pattern on the material. For example if you are going to have to match up strips etc you are going to need more material. Personally try to avoid doing that. Working with large slip covers and long long zippers can be challenging enough without worrying about matching it all up.
Interior designers more frequently than not will also suggest you go with a solid fabric anyways and simple dress/jazz it up with pillows & curtains with lots of pattern that are much easier and cheaper to make. That way in a few years if you decide you no longer like the pattern you can easily change them out.
If you plan to do a edge trim you can make it yourself out of the same material so you will need to allow for additional material for that and buy the cording on a big roll but be warned there are miles and miles of trim on the edges of the original covers and you will be spending a good day or two just cutting out and making enough matching edge cording.
Look for a shop that sells home decorating fabrics. They will usually sell zippers on a roll that they will cut to the lengths you need and have all the little parts you will need to put them together - i.e. install the stoppers etc Its not hard to do. I measured all the zippers on my scamps cushions and actually added a bit of length to a couple of them to make it easier to get them on and off.
When picking a material look for fabrics that are actually designed for upholstery and home decorating. The indoor outdoor materials such as Sunbrella also work but I have found for seating that they do tend to sag/stretch out a bit more than what I would like. Upholstery fabrics are heavier (so keep lots of fresh sharp needles handy) and will have very little and often no cotton content. You want something that will stand up to the wear that seats are subject to.
Cotton does not stand up will - it fades with wear and sun so you should not expect it to last for long and still look fresh and new. Not to mention cotton will absorb moisture - something you DO NOT want in a trailer.
Its a big job & time consuming to do the cushions well, so its not something you are going to want to do over and over again. Trust me you will be asking yourself more than once "what was I thinking" ;-)
Look for a fabric that can be washed - most upholstery materials will not do well on a regular wash and few will go in the dryer. But many of them can be washed on a cold hand wash cycle and hung to dry. Talk to the sales staff about the content of each to make sure it can withstand a hand cold water wash once a year or so. The good news is if you get a good quality material that does not absorb moisture and smells, you will not be needing to wash them very often.
When using a good quality home decorating fabric you do not wash the fabric first. Best to sew it as is so it retains its sharpness and shape and any stain resistance it may have on it. Due to the lack of cotton you do not need to worry about it shrinking.
A word of warning. Good quality upholstery material that will withstand the test of wear and tear is not cheap. Once you add up the materials, rolls of thread, needles, cording and zippers (days of your time not included) you are going to be paying far more than what you can purchase the pre made ones from Scamp for. Scamp's seamstress is purchasing everything in bulk so they are not paying what you and I would for all that is needed.
Scamp also uses or at least they use to, a very thin plastic covering over their foam cushions. You will want to save that as it is hard to purchase the same thickness of plastic they use and its already made to fit the foam. You will need that plastic on the foam to get the covers back on the foam nice and snug fitting, not to mention it helps to keep the foam nice and clean (does not absorb any moisture that may get spilled on them).
But once you have have taken the time to do it well, you will be happy you did, as it will be uniquely yours and stand up to time.
Salerite has some great very detailed step by step Youtube Videos on how to make a Boxed cushion.