I have a 1978 dodge 440 in a motorhome and the engine turns over but as soon as I let off the starter it dies. It seems like its not getting power to the engine?

It could be many things so start with the easiest /cheaper first.Check your plugs , they will tell allot...white= too lean .black and/or wet =too rich .. clean and gap If your coil is old and not sparking hot enough it will sputter and stall . If your ground to the ignition is loose or corroded , same result. If the motor has noit been run for a long time ; replace spoark plug wires , coil , pioints, rotor , cap ( just replace the distributor with electronic etc.) . Put in fresh fuel (gas goes bad) and filter.You might run some carb cleaner or rebuild the carb , your start /idle jets might be plugged.Check all vacume conections , cracked or loose hose equals too lean to run when cold. If all else fails loosen the distributor and swivel small increments back and forth until it lights. Oh yeah , if your alternator is not putting out the motor will kinda start then die beause the battery cannot keep up.
Good luck