Venice may not seem like the obvious choice of a place to go in your motorhome but I have done it on more than one occasion. Obviously you are not going to get a view of the Grand Canal, but you are going to see it. The trick is to park somewhere not too far from a railway station such as at Venezia Mestre and then take the train in. It will work out much cheaper than parking at Piazzale Roma!
In my opinion, Venice is best in winter although riding around on the vaporetti (boats that are the same as buses everywhere else) is not as pleasant as it in summer. That apart, Venice has a charm of its own out of the tourist season!