Mine is a 1973 1300 same as yours, but for some strange reason it was originally built with the water reservoir on the kitchen side and the
furnace was on the curb side. I used the original DuoTherm exhaust fitting on the curb side, but had to modify it slightly, because on the Atwood in and out are not coaxial, see picture. Also the exhaust vent is in a different location.
The rest was just mounting 3 4" vents inside and insuring there was sufficient cold air return. I mounted it with angle brackets to the original
furnace platform.
It is a larger unit (30,000btu) than I would have chosen had I choice (found it on kijiji), but I'm very pleased with how well it works. It just runs on a lower duty cycle than a 15,000btu unit would, and in 2 minutes the place is warm. It draws 6.7A while on, nothing while off, so the lower dutycycle means it is on half the time a 15,000btu unit would be on, so average current consumption is half. Heats faster, Watt/Hour consumption less.
It's sweet.
Doug