We have found plent of room in our Scamp for clothes and just about everything else we need for our 11 months. We use no suitcases, duffle bags or backpacks. We have added storage every where we could while keeping a very open feeling in our Scamp.
Though I am very familiar with a Scamp 13, having modified our son's 1977 Scamp 13, I do not know your lay out. His Scamp 13 has a permanent bed in the back and a 2 seat dinette in the front/ converted from the bunks.
If you have the bunks in the front and have left the couch the way it is there's a lot of room under the couch for storage. Here is what we do.
Under couch center compartment: Two plastic bins for Ginny's Clothes. Behind Ginny's bin there's a space for two shoe boxes for tools.
Door side of couch, cut out front of fiberglass and installed a large drawer for Ginny's clothes. Behind this drawer I keep some rarely used tools, collapsible saw, soldering iron and wire.
Driver's side dinette, installed two drawers with plastic bins for my shorts, underwear and socks. In addition to these drawers there's space for a cube heater, fan, slower cooker and a lantern.
In our son's Scamp he keeps the bed made up. Under his bed we installed two full depth drawers. Each drawer contains two plastic bins.
Over the dinette/now bed we installed a full width overhead cabinet for his clothes.
In his Scamp we removed the couch and built a 2 person dinette in the front of his Scamp. Each dinette seat has a large drawer that opens to the center as well there is storage under the seat itself.
Over the 2 person dinette there is a shelf for storage and above the two person dinette table, built on to part of the couch back support is another small shelf.
We also shelved his closet.
For all the misc storage stuff, we added a bumper box on the rear bumper that containes hoses, cords, blocks and jacks. Under the bumper box are two 5x5 inch plastic fence posts that hold poles for his clothes line and awning
In our Scamp 16 we store our pare of back up shoes under the edge of the side bath.
Under Scamp boxes like I added to our Scamp 16 are not possible on his Scamp 13 because his trailer is much closer to the ground than ours.
You can see pictures of our Scamp under modifications/Preparing a 1991 Scamp. If you want more pictures just let me know.