Hi Carol,
We have done Calais to Rome, to Florence and to Lake Garda a few times so whilst I don't know the final part of your journey I am pretty familiar with most of it.
First, the mountains - shouldn't be a problem at all unless your car struggles already with your van on lengthy inclines in the UK. For the most part, the mountains you will meet will be in Switzerland and as you will discover, the Swiss have a habit of driving through mountains rather than climbing them in their cars. There is of course a long (but not steep) ascent up to the St Gottard tunnel - and an even longer one on the way back home but very few outfits I see in the UK would find it a problem. Stand by for some lovely scenery. Travelling over the Italian central mountains was no more exotic or difficult an experience.
Second, two days and 1087 miles. Well it is very doable, depending upon your driving arrangements and stamina. Sue and I have a system where we drive for an hour each in turn. That way we once drove just over 650 miles in a day coming back from Lake Garda with the van and two very good children (that day). The roads are certainly good. If you are very unlucky, the queue for the St Gottard tunnel can cost you anything up to 2 hours in the middle of the day in high season - but in May you should be OK. There is the tricky problem of speed limits - as Lutz might have reminded you, after France the limit is 80kph (50mph) for a car and caravan in Switzerland and italy. However, in practice if you drive at that speed you will be the slowest vehicle on the road by some way. Most continental outfits seem to drive around 100kph. I offer no advice on the matter.
A couple of stray thoughts.
Buy your vignettes for the swiss motorway system in advance - you will need two, one for car and one for the van. You can order them online from the Swiss Travel Centre -
http://www.switzerlandtravelcentre.co.uk/vignette.html
Service areas in Switzerland and Italy are spaced much more like the distances in the UK rather than the more frequent aires in France. If you stop in Switzerland you will find them very happy to take your euros (at probably not a very good exchange rate) but you will be given change in Swiss francs.
Have a fantastic trip,
Mike A