I am replacing a Xantrex SW 2012. The Xantrex has one hot, a ground and a neutral, not the two hot legs I expected. Do I need to rewire with two hot legs to the AC distribution panel or can I connect with the one hot leg?
Do you have 50A shore power? It sounds like your RV was built with an inverter loop rather than the inverter powering the entire load center. If you have a 50A load center and want to energize the entire panel, you may need to replace it with a two-buss load center or at least rearrange the circuits. And, you will need to run new cables if you want both legs of power. 6/4 SOOW cord is a good choice for this. If what I said isn’t clear to you, then please contact your local Victron installer or distributor for further guidance 
50A Shore power and a 7K gen set. Though not optical, would it be OK to use the one hot leg wired as it is?
Definitely. We sometimes use the 2x-120V on 30A installations with a single hot leg.
So, are you replacing the Xantrex with the identical Xantrex, or are you using a Victron?
I am replacing the Xantrex with the Victron MP II. I traded in our previous motor home (30A) and was able to keep several Victron components (Cerbo GX, Touch Screen 70, MPPT solar control, and other items. Our new MH is 50A and I thought the MP II 120x2 would be a good way to take advantage of what I already had.
Like Ed said, the Xantrex is apparently not installed to power the whole house(RV). It was likely set up to power only one side of the 50 amp service or possibly even just a single circuit or sub panel. If you want to power the entire house and fully utilize the MP 120X2 you will need to rewire with 6/4 cable to the power center.