Hi Mike,
you must have read my motto, "Success consists of going from failure to failure without loss of enthusiasm" .......(Winston Churchill)
Well, that's me. I'm afraid I tend to work on the theory that if I read the book I can do it. Not as well as some, not as quick as some, and not always right the first time, but, great satisfaction when its nearly right, and I did it.
I'm not using all new bits. My original roof vent in "Lightning" (teardrop) was a damaged, Omnivent unit, with a fan. I was going to fit that but a new unused unit came up on ebay, and I managed to win it. So that's going in now. The old unit might move down the camper to the cooking area, at least to start with. The beauty of it is the fan unit. I can buy a new basic vent unit and my old fan will fit in it.
They are quite easy to fit rearly. When the screws are taken out on the inside, the pretty bit falls off and, with a bit of luck, you can ease the outer frame off the mastic and lift it diagonally through the hole. You don't even have to fall off the ladder
The 400mm square hole is big enough for me to manipulate my way out, so, with something inside to stand on inside you can play tank commanders
, while cleaning the old mastic off.
The fact that the vent inner and outer frames screw together within the 400mm hole and sandwich the roof makes all possible from the inside. Also the basic unit from motorcaravanning.co.uk is about £74 but you can buy the fan assembly later. Unscrew the pretty bit with its screen and blind in it put the fan in then all back togther
simples
Go on
you can do it
Mark