Truck won't stay running , no headlights.

I have a 1990 e250 , the problem started with the
truck slowly stalling after pulling out of the parking
lot. Then little to no power to interior lights.
Checked battery it was bad. Also the positive
terminal conector was black. Replaced positive
cable as well. Now still low power to interior lights ,
radio etc . No headlights. Once I switch the
headlights on everything inside dims like there is
not enough juice . Checked voltage and battery is
at 12.4 as well as the terminal connectors. Vehicle
will slowly start and die or will not start at all. Seem
when I start it with starting fluid it stays on for a little
until the fluid runs out . Still when running the
headlights will not turn on . I also replaced the
alternator a couple of months ago and starter
solenoid recently. I give up trying to diagnose this
issue. Any ideas what could be the problem? The
truck is so old I don't even have a diagram for any
fuses or relays . Not a OBD1 reader. Help will be
great appreciated.