Depending on where your batteries are located and where you want the DC power outlet, you should be able to find a good location for the power socket. For example, a relatively dry & protected location would be under the wing over the pickup box walls that can be easily found & reached to insert plug in the dark. If on the awning side, you might also consider replacing the awning lamp with a red LED bulb or installing a red LED strip near the outlet. Make that circuit switched and turn on the red LED strip when you power the socket.
Choose a power socket compatible with the power plug on your telescope or use one that is different from the rear wall solar receptacle so no one plugs a portable solar panel into the wrong spot. Be sure to include an inline fuse close to the battery.
A "no drilling through the camper" possibility is to modify the porch light to allow plugging in the telescope power cable to the porch light. It is already wired and switched. The lenses are readily available so mount a power socket in the lens & you're done.
Paul