I will be visiting in Scotland for the month of July 2018, traveling by car. I plan to visit the Orkneys, Skye, Lewis, Iona and Mull. I'm wondering how hard it is to get ferry reservations for a car in July? Not sure how often they run in summer and what their car carrying capacity/trip is? How far in advance I should be reserving a spot for these different trips?
Did a trip a couple of years ago through the Scottish Highlands - Glasgow, Oban, Iona/Mull, Orkney and back down to Edinburgh. Trip was in September. Since you are traveling during the height of the tourist season, I highly recommend you check the Cal Mac website, choose the dates and times you want, and book your tickets - especially if you don't have much flexibility with regard to your itinerary. If you are flexible (i..e don't need to be at any of your destinations on any specific day then you can wing it. Anyone without a reservation can get in line and if there is space, get on the ferry. As for our experience, every ferry we took was completely filled - now how many of those had reservations I couldn't tell you - but the ferries did not have any excess spots left.
The longer the ferry, better get a reservation. All are theoretically drive up routes, but on some routes you can wait long time. Except Iona which except in specific cases is pedestrian only and basically a bus.
Apart from Orkney, these ferries are all Cal Mal.
We have always made reservations for the Scottish ferries, usually a few months ahead. If you are unsure which sailing time you want, pick the later one. We have always been allowed to take the earlier ferry, even if our reservations were for the later one. The Orkney ferry is much fancier than the Cal-Mac ones, but also a lot pricier.
For a car-tripper, there aren't a lot of options on Mull. It seemed more like a place to visit if you wanted to do serious hiking or birding. There's a road around the perimeter which is very narrow (even by Scotland standards!) on the southwest side and a connecting road at the middle. There aren't lots of side roads like there are on Skye or Lewis. If your main interest is Iona, you could take the bus from Oban.
Car places on the Cal Mac ferries to Lewis are especially in demand in July, as are accommodations on Lewis. calmac.co.uk shows the summer timetables, and bookings are already open. There are 2 routes Stornoway-Ullapool, and Harris-Skye (Tarbert-Uig). Each route has 2 sailings in each direction per day. Stornoway-Ullapool would be convenient if you're coming from or heading to the Orkneys.
I would also start inquiring about lodging on Lewis now, particularly if you want to stay on the west coast around the breathtaking Uig area. And then book the ferry as soon as you have your room. (For my Lewis/Harris/Uists trip last year in late May, I reserved my b&bs in January and found some were already fully booked.)
Have a great trip!