Number one - get a road service! We have used Coach Net and have been very happen with them. That takes a lot of the worry off as they will send help and provide a tow if necessary.
We do carry an extra fuel filter and know how to change it as a bad load of fuel is not uncommon. It is changed annually with the oil so unless we get bad fuel, the spare isn't used. They go bad sitting so we switch the spare out with a new one and put the spare on during the annual service.
When the coach was six years old we changed the fan belt and kept the old one for a spare in emergencies.
We have never used the spare filter or fan belt but you never know. I am not a DIYer and perform none of the maintenance myself, thus it cost us more than those more handy. Just make sure the annual maintenance is performed and what needs done goes through a three year cycle so something new is needed each year. I carry no special tools for the diesel I didn't have with our gasser.
