Hi Dieterdog:
Trailer brakes would be the easiest and least intrusive addition; have the old axle replaced with a brand new axle that comes with electric brakes.
A potable water tank could be added somewhere out of sight or hidden. Or you could fill a 5 gallon plastic jug with drinking water and store it under the kitchen sink. Drop a small hose in it from either a hand pump or a
12 volt pump (but then you'd need a
12 volt battery, etc.) to get the water to the countertop.
I didn't think this early a trailer came with a toilet. Is there one there? If not, consider using a porta-pottie that could be stored in the closet or otherwise hidden from view. If a toilet is there, then adding a black water holding tank would depend a lot on the toilet location and how much support would be available for the black water weight. The chassis frame may, or may not, have to be strengthened to handle the extra weight, depending on tank location and capacity.
Instead of a grey water tank, how about just placing a 5 gallon plastic jug under the kitchen drain downstream of the sink trap? An alternative would be to use an exterior 'blue boy" water tank at the kitchen sink exterior drain.
It would be far more difficult to add a
12 volt electrical system. Propane appliances (hot water heater, furnace, refrigerator) would require some thoughtful work, as would other conveniences we take for granted today.
If the trailer is great original condition, and you are not excited about adopting yourself to the trailer, then you might consider passing it along to someone who would use it as originally built and look for a slightly newer trailer with more modern conveniences for yourself.
Good luck with your decision and the trailer. Please keep us posted. Pictures would be enjoyed.