I'm not in the States, Tele, and we have different brands here, but look for a detergent that allows you to soak the clothes overnight in it beforehand. Then, the next day, when you wash, the stains/dirt/grime come right out with little effort.
In the "Mexican" sections of supermarkets, or in mini-markets in Latino neighborhoods, they sell bar soaps that are good for washing clothes on a washboard. The soaps rinse out very easily. Don't use a brush on the clothes unless they are very heavy duty. A brush will wear out the clothes fast. Just grab a fistful of the cloth in one hand and gently work it back and forth across the ribbed washboard surface. Turn the clothes inside out before you do this if you want to preserve the outside, especially if it's a T-shirt or something with some kind of a logo or design. Make sure you keep the washboard free of sharp or hard particles that could rip open a hole in your clothes.
I have a slow connection or I would do it, but I bet you can find some web sites with helpful tips or even YouTube videos with advice also.
Also, wash out the day's clothes that same night. It only takes about 15 minutes and that way a ton of dirty clothes won't get on top of you all at once!