I am not sure how the fuel pump for gen is plumbed, but wondering if you could remove one of the fuel lines on fuel pump and put cutoff valve there with suitable adaptors. Just a thought..
Our MH is new and is not the first ,the gen set is smaller than our last one 2011 model. I have read the manual from Onan and it only says to prime the gen only if it has been run out of fuel. We have the 2501ts Froester I haven't had to use the prime switch yet.