1986 390 carb engine. Pour gas in carb only way to start
 

This carburetted engine in 31 ft allegro motorhome. Not run in ages, gas tank full of rust but no holes. Electric fuel pump just outside tank, followed by clear plastic filter. Believe also has orig mech pump on engine.
My goal is to get it started to move 100 ft out of my way. I disconnected rubber fuel line from tank where it connects to metal line on engine. I also disconnected power to aft pump and blocked off rubber line from tank, at engine.
Next i hung a lawn mower gas tank on front bumper, level with ctr of engine. I added shutoff valve and new filter, then new rubber line to rear of engine where i connected it to the metal fuel line on engine.
Fresh battery charge spins engine fine, builds up oil pressur like normal, but wont start. If i pour some gas in carb, reclose breather top, it fires right up, but of course, wont keep running.
Last time i actually messed with this engine was years ago. I'm much older now, dont do well bending and stretching any more, so need some friendly encouragement and a direction.
its obvious gas is not getting into carb in normal way. Need help visualizing whats in between rubber supply hose and the carb that might be gummed up.
ive always used stabl stabilizer and/or seafoam in tank. Im hoping i can get by just forcing cleaner into the lines. Heck, has worked well on lawn mowers!
so, folks, what do you think? Really would have trouble opening or replacing the carb if not absolutely necessary.
thanks, if you can help, Tom

Assuming you have the means to do so, I would start by trying to blow out the remaining line used after the system reconfiguration to the lawn mower tank.

Do you have a fuel pump between the lawn mower tank and carb?
My take is you don't have a pump hooked up.
I don't know what that hose is you are tied into at the back of the motor.

What size motor are we talking of here?
Is the 390 the carb size or motor size.
If motor size then you are kind of on your own as 86 trucks did not come with 390 motors.
Now I can see a 460 motor and I think the 460's came with 2 electric pumps. A low psi suction pump in the tank and a high psi one on the frame to push the fuel to the front.

You also said you think it has a pump on the motor. If what I said above it true then no pump on motor. You will have to make sure it does or dose not and if yes hook the fuel tank to it.
You have to have a way to pump the fuel to the carb.
Dave ----