400 lbs is about 48 gallons of water. Are you talking about (3) 400 lb bags? Because a 1200 Tsunami is rated for 20 GPM, which would would fill a 400 lb bag in about 2.5 minutes. I believe you did the right thing by having a seperate intake for each pump. You can only get about 38-40GPM max out of one 3/4"-1" scupper.
I am currently using (3) 800's off of one manifold and one intake. I can fill 1900 lbs in about 9 minutes. I was considering trying the 1200's to speed this up, was not sure if the 1200's are actually producing 20 GPM fill times. I am getting about 25-28 GPM with three 800 Tsunamis, not too bad but would be nice to get a little more flow with the 1200's. 1900 lbs of water is about 228 gallons of water.
I dont know why Atwood did not make the 1200 more user friendly with standard NPT 1" or 3/4" thread. From what people are saying its a pain to adapt the 1200's to 3/4" NPT.
Did you have any problems?