It's likely your camper may not have had 120 vac in it originally.  Those that did normally came with a single 120 vac 15 amp circuit and entered the camper somewhere on the side. The plug was connected to a fuse and one circuit in the camper.  You connected an extension cord into the plug and had electric inside your camper. This worked ok because most people didn't use alot of electrical appliances when camping; likely a light and maybe radio, etc.  The outside plug normally was covered to help keep the elements out when not in use.  The bad part:  once you removed the cap and plugged in the extension cord then the cord plug was open to the elements.  Since they didn't have ground fault systems back then to shut off the power if it got wet this could create a hazard.  Today, with campgrounds having ground fault circuits this isn't as much of a hazard as an inconvenience.  Even so, if you wanted to replace it with something like an outside box and cover, this isn't much difference as you still open the cap or cover to plug in the cord.  But, you could replace it with an RV electrical hatch like this one or the door one on down the page.

The snap lock vermin door will accept up to a 50 amp cord.  With these you open the door and pull out your power cord and then snap the lid closed and the 'little slide in the middle' can be moved to make a tighter fit against the cord.  Of course with this you would either be making your power connection inside or the power cord would be connected to the main power box and you just pull the cord out. This is the way most new campers work.  These are not that expensive, around $10 - 15 and are available at most RV stores. This model states:  These RV electrical hatches have a low profile, attractive design and are UV stabilized to protect them in most environments.  Use them to protect your cords and store them away from the elements.  These are fade and impact resistant and should provide years of use. Easy installation. Snap lock vermin door will accept up to a 50 amp cord. Cut-out size: 3-1/2" diameter.OD: 4-1/2".



This is an idea on how to convert your old power connection on the side of your camper.

1. First decide if you just want to replace the old power connector with a new hatch and run your cord inside to connect or you want to eliminate it and use a key hatch?
2. To just replace your old power connector is a simple procedure.  Unscrew it from the side of your camper.  Then remove the wire connections and toss in the trash.  Next, measure the opening and it will need to be the size of the new hatch opening, the one above is 3 1/2" diameter.  Once you do this, then make sure the new hatch has a gasket or sealer.  Then place in opening and screw in place.  Now you can either run your power cord inside and connect or make a permanent connection and store the cord inside with the hatch closed.
   
3. This is a key lock door hatch for added security.  It will be the same installation except the opening is a little     larger.  If you use this instead of the other one above then just make the opening larger and it will install the same as above. Or maybe you want to put it in a different location.  A couple places to consider would be into the area under the dinette seat or the area where the water tank would be if you don't have one. To me, I think the area under the dinette would be good if you just wanted to store your cord there and run it to a main box somewhere above? The water tank area is accessible under the bed usually by removing the plywood.