Yes, the Winegard 1518 can be mounted on the roof. I did it. I must say it was not my idea... the method and pictures did exist on this site but that was a couple years ago and, by the time I got around to it, the pictures were no longer there but the text was. You might find it if you search.
Anyway... I'll try to walk you through what I did. I had one of the old (obsolete) mounted dishes on the roof that didn't work. I removed everything but the base plate and wiring sticking out.
The tripod for the 1518 can be purchased more cheaply on ebay, if I remember right. I found stainless L brackets (online) that had two uprights spaced so that the bolt would go through one side, through the two holes (where the legs used to be) and through the other side. I wanted to make sure my mount was rigid enough... I hate surprises going down the road. ๐ Mounted the brackets to the roof with stainless screws and caulked with lap sealant. So... what I have is a removable base plate (to get under so I can recaulk in the future) that holds the Winegard 1518. There are three slots in the base plate that the feet of the 1518 fit into and rotate to lock in. Ok... "lock" isn't really true. It holds it down but nothing would keep it from turning back out so I use a bungy from the 1518 handle to one of the brackets. It doesn't take much pressure, just enough to keep it from vibrating out of the channels.
On to the wiring.... I bought an additional power cord that has the Winegard connector, cut off all but the connector and wired it into the existing wiring from the old dish. On the inside of the coach, on the box that held the internal controls I put in a toggle switch for the power. When I decide to move, I flip the switch so that the dish won't continually seek. No... I have never tried it while moving. I don't believe the auto seeking abilities of the 1518 are up to the task of that rapidity.
So... what I have is a 1518 on the roof when trees are not a problem (and also storage when moving). Should I park somewhere and have a tree problem, I just unplug the power connector and cable, release the bungy from the handle, give it a turn and I'm holding it in my hand. I use the original power cord and cable wire that come out of the electrical bay and put it where I need to get a signal. I have the best of both worlds. Even counting the tripod and connector wire, I seem to remember having less than a hundred bucks in the job. It has been a couple years and a several thousand miles with no troubles.
I could take some pics and send them to you but I've never bothered with figuring how to post them.
