Just read through this thread and i'm guessing we need to travel in something comfortable yet cool, but take a fleece / coat / blanket with us encase it gets cold on a night and for the service stations. We should also take packed lunches with us to save on the cost of the French service stations. A book or magazines is a good idea, but anything electronic might need charging and that might prove difficult.

Are the toilets on the coaches ok for the larger person ? And how often do the coaches stop ?
Oh and what sort of food and drinks do they sell on the coach ?

BTW we've bought a portable electronics charger that can charge a mobile phone fully about 3 times on one charge of it's own. We thought this might be a good idea so that we could mess around on our phones all day and night and still be able to power them up.

we used the powermonkey phone charger on holiday, which was a great buy as I had managed to leave my mobile switched on accidently and found at the airport going home that it had drained. It charged my mobile fully and also my husbands later on and still had some charge left

A mobile power charger is a very good idea, next time I will get one. The only reason I didn't was because I planed my coach trip at the last minute so had no time to go shopping other than the essentials.

I can only talk about Ferris but some points which might help but some of this depends on the weather.

I mostly found when I got off the coach at stops the heat me, even in the middle of the night. The air con on the coach means it can be a bit chilly. I most wore a jumper and a jacket on the coach.

The toilet is a bit cramped no different to the toilets on a bog standard coach.

The toilet stops were about every 5-6 hours, but we missed the ferry on the way there and had to get a later one, so maybe there is usually more stops on the way there. I found taking diaerea tablets helped so I didn't need to do a number two on the coach (I suffer from IBS).

I have a weak bladder and the toilet aspect did bother me, but I found if I went to the toilet at every stop I would only need to go on the coach once inbetween the stops. The aircon though made me need the toilet more often due to the chill.

I didn't have any drinks onboard as I bought my own, but it seemed to be the usual tea, coffee or canned drinks.

A pack lunched for the journey going is a very good idea, but would be harder on the way back, Ferris do not allow you to bring a cool bag.

My friend & I have been to Ostend a few times on the coach, not as long a journey as one to Spain, but here's a few things we take.

Kindle as its a lot lighter than carrying lots of books.

We take sandwiches, but also take those pasta & chicken meals you can get from supermarkets as they are really filling, tasty and are a change.

Wet wipes in order to have a quick freshen up.

A mosquito repellent roll on may be handy too if the coach stops during the evening, as the little blighters can easily fly into the coach.

Comfy footwear that is easy to slip on and off.

Mints to keep your mouth fresh.

Food on the coaches is usually reasonably cheap, As you don't stop for long and the queues at French service stations are horrendous id recommend avoiding using them if possible. Price wise they are similar to UK service stations (ie extortionate).
We usually take "hoody"type zip up jackets as it can get chilly on a night