I recommend you do not put an electric pump on.
Much of the time the problem of losing fuel in the lines is due to a few issues:
1) the fuel lines could have collapsed due to crummy ethanol fuel or could be dry and brittle- replace them all. you may want to do this every couple years unless you use fuel injection line, or at least put good ethanol resistant line.
2) the tank could have a blockage in the outlet
3) you absolutely NEED clamps on all the fuel hose joints or you will get air leaks and fuel will drain back
4) GET A BRASS FLOAT - it will save you headaches so often - the foam floats swell with moisture in crummy ethanol gas and will stick closed, or stick open and cause flooding.
5) use a sintered brass easy flow through style fuel filter
6)You can add a one way check valve that will keep fuel up near the carburetor so it should start quicker.
7) Clean the carb well- I am talking using hang tag wire to clean the idle circuit out, and make sure the carb cleaner will spray though all the passages freely.
8) Have Patience. When you get them to run correctly, it is so worth it!
I have rarely ever had to buy new parts for onan carbs. they get a bad rap mostly because i think people lose patience with them.
Most times, keeping them clean, and rubbing the diaphragm with transmission fluid or motor oil to soften it up will keep them running for a long time.
If you want to use an aftermarket pump, add a briggs style pulse fuel pump it will work better than the onan one.
You basically run you fuel line from the tank into this guy, hook your pulse (vac) line up to this, and the output of this pump goes to the inlet of the Onan fuel pump.
That works fine, but when I switch to a newer style pump, I try to (and I stress try to) find a onan pump off a 318 john deere, and use the fuel input plate from those instead of the pump plate on the onan.
this style john deere pump- you basically use just the inlet fuel plate, and take all the pump stuff off the Onan DD carb
this setup for me has worked the best
not sure if I forgot anything, but if I did, I will add more info.