When you buy a used travel trailer or your own camper is getting a bit long in the tooth the first step is to look at the colors you have to work with. RV manufacturers generally decorate with a theme of some sort. Are your colors in the creams, or blues or greens? If you are able bring a pillow or head rest cover that will help you pick fabric for your curtains.
With color samples in hand head to your nearest fabric store or online to an RV after market supplier. The fabric store will have a wider selection of colors and patterns but the online RV supplier might have fabric to match what you have already. Purchasing pre-lined fabric will save great amounts of time with this project. Thin fabric might have a pretty pattern but when the sun shines in it will give no protection or modesty to those inside. Imagine sitting out on your RV patio only to glance over and see right into the bedroom of camper next to you courtesy of thin fabric curtains!
If you have them use the previous curtain as your guide in how much fabric to buy. Most used motorhomes come with their old curtains and while you might not want to use them while rving they are the perfect way to figure out how to make your new rv curtains.
If you don't have any old rv curtains to go by measure the length and width of each window to cover. Consider the rod and any extra hang length you want. Generally you will want to double the window width so you will have fullness to the curtains.
With the fabric and measurements at hand sew a rod pocket on the top and a hem on the bottom. Slip them back on the rod or install spring mounted tension rods sized to each window.
Easy no-sew curtains can be made by using fuseable web, an iron and clip on curtain rings. These are fast and easy and look remarkably nice and are easy to open and close.