Hi,
The 1978 converter is the real problem. However it is somewhat good news, as the converter can be run without a battery in the circuit.
A modern solid state converter/charger will have less hum, but also requires a battery to run the system without power spikes.
You might try a small battery to run the radio. Wire it up so that it can charge from the converter for say 2-4 hours, then wire the 8-track directly to the battery. THen turn on the timer when you want to charge the battery, such as when you are leaving the trailer, and shut it off when you want to use the radio. If you use a switch to charge the battery, and leave it on to long, then the battery can boil out all the water in a short time - the battery might only last about 3-6 months.
If you can buy a cheap 12 volt battery, even a used car battery, or something like that, you can solve your hum problem.
Fred.