Good catch, I missed the question on the cord end. It cannot be original, even the 65 Safari I had came equipped with original cord, 30 amp, and it was what was shown in the owners manual.
That could be more of the issue than anything else we have suggested. I would start with the cord. Who knows how much the spliced on end is creating in resistance. I think we all assumed that the cord had a 30 amp end.
The easiest way to replace it is to buy a 25 foot 30 amp extension cord from the local camping place and cut off the female end. You will need to follow the cord into the panel on the trailer and once again follow the wire colors. Depending on the local Home Depot they may have 30 amp RV ends. They run 10 bucks or so. Before you install it though, be sure it fits through the hole you feed your wire through
this is the voice of experience speaking
This is something I usally have to do on every unit I buy. Ends get frayed, poor campground outlets overheat and cause the hot leg to melt in the end, wire spliced by PO with wire nuts and electrical tape. You get the picture.