Firstly:
There will be no Tory/UKIP coalition after the next election. To form a coalition both parties need to have some seats and UKIP simply won't have any. Unfair perhaps, given that they will probably poll quite a few votes, but the fact is: they won't. Added to that the fact that the only scenario which WOULD see them winning seats would be Tory electoral meltdown, in which case we'd have a Labour landslide anyway.
As to Cameron's position on the EU, it is, honestly, farcical. He is onto a complete loser with it: his proposals will not impress the hardened Eurosceptics, will at the same time alarm Europhiles, and will in any event be laughed out of court by the EU. You can't be sort of in the EU and have the benefits without any of the costs.
Because of issues like immigration (exacerbated by hard economic times) the EU is not very popular at the moment and UKIP are as a result doing quite well out of this mood. Cameron of course is aware of this and is trying to appear more of a Eurosceptic to draw their sting and to appease his own Eurosceptic wing. But, like I say, it's a policy doomed to fail.
There can only be an EU referendum if there are some concrete proposals to vote on. there will never, trust me, be a referendum where there is a simple "In/out" option.
Labour are likely to win the next election, and the hope will be that better ecponomic times will reduce the effect discussed above. Labour do not have a large Eurosceptic wing (how times change from the 70's!) and so Mr Miliband won't have this problem.
In short, Europe, in my opinion, is more of an issue for the Tory party than the country.