Affording it, affording it and affording it - to answer your three questions. This answer coveres a number of problems which I cannot see being resolved by another 'good' or service, but appreciate your initiative and would be very happy to be proved wrong.
The fact of the matter is that the main costs of European touring - ferry, fuel, insurance, campsite fees - have all risen considerably more than the rate of inflation over a number of years, whereas incomes of all types - regular wages, pensions, investments have fallen against the same index.
Insurance is also age related, and as the average age of the population is increasing these costs have increased disproportionately.
Then the general costs of living have also risen way above the rate of increase of income (even, despite some newspapers today0 for those of us above 65) and travel generally has to come out of disposable income rather than standing charge provision.
A further complication is that for many of us who winter abroad, there are savings in the way of reduced heating and electricity and water costs, but these are very difficult to quantify as we never know how much oil we may have burned if we had stayed home. Generally food and household costs are equall or below what we would have spent at home - unless of course, you eat out a lot.
So target the parts of the sum which are unique to touring - ferries and campsites with insurance as a follow-up. It is very unlikely you can influence the cost of fuel very much.
Here are two sets of figures. Both relate to recent stays - first in Spain of 116 days (including transit through France each way but Western channel Crossings, the second to Provence for 42 days using Eurotunnel (twice as far from my base as the ferry).
Spain. Ferry 17% Insurance 10% site fees 44% fuel and tolls 29% of total cost. No allowances for UK savings on heating etc. Food costs assumed equal.
France Eurotunnel 14% insurance 8% site fees 33% fuel and tolls 45%
I am composing this on the fly and it is now clear that insurance is not such a major factor after all. Ferry costs will be heavily influenced by where you live, time of year of travel etc. In my example Spain is Decmeber to April, France is September to end October.
I really regret having to be such a wet blanket, but I believe it better to be realistic. The number of people how have been sucessful in caravan related ventures is pretty small, and most of these are not european touring specific - e.g. motor movers - unknown until about 14 years ago, or frresh and waste water containers where one brand name has become almost the generic. So I can only suggest you concentrate on something applicable to caravan touring in general and not European touring specifically. I would also suggest a product which does not have a high after sale burden (service, warranty etc.) I have no idea what this is or I would be making it myself ! Good luck !