We freecamped at Iznik last year by the lake (40.42310 29.70907).
Can't help with the ferry, all I can say is we made the mistake of driving around using the backroads from Iznik to Izmit, the road surface was terrible with potholes all the way until we picked up the E80 into Istanbul so don't be tempted to try that.
Do you have somewhere to stay when you get to Istanbul?
We are in Izmik by the lake right now. Lovely spot. Think it was your post that led us here but we didn't have co-ords so it took us a while to find.
The 150 from Iznik to Mekece was rather rough but being rebuilt.
We are hoping the road along the northern shore of the lake, towards Yalova will be a good surface. We are coming across brand new roads all the time.
We are hoping to roll off the ferry and be very close to the Kennedy Cad parking area for Motorhomes? Do you know any other nice spots we can try? We plan to be in Istanbul for 6 nights.
Grath, We have done Eceabat to Canakkale and just rolled up also.
There is another option further along Kennedy Cadesi by the sports complex with showers etc.
You will be on the right side of the road (heading west) from the ferry, we came in over the Bosphorus from the east and there are no gaps to cross along the dual carriageway so we had to go a fair way past before it was possible to double back.
Magbaz has some further info about both stoppovers on their website...
We did Istanbul to Yalova a lot of years ago and then to Bursa BUT we were on push bikes so not much help to you, sorry just couldn't resist telling you. We love Turkey and would love to tour it with the MH after about 10 trips with the bikes.