If your TT doesn't already have a bypass valve installed on the intake side of the water pump, get one of
these and install. This will allow you to bypass the water tank and suck from the RV Antifreeze jugs to push antifreeze into all of the lines. With the siphon hose in a jug, just open a faucet until pink stuff comes out and close it. Do each one (hot and cold) including the toilet, shower(inside and outside), all sinks.
As far as the leaking roof, a yearly inspection should be done on the roof and any place else(Awning brackets, lights, windows) where water can get in and sealed up using Dicor(not any other caulk especially not silicone based). Especially if it sits outside year round. My guess is that maybe the previous owner stored it under cover and it needed sealing before being exposed to rain/snow.
Sorry to hear about the water damage. I've had to replace a roof on a motorhome and it was quite a project. Wouldn't do it again. Good luck with this.
