I agonised over the options for this for two different trips a couple of years back. Albeit on a motorbike, I'd imagine the comparisons / contrasts between the options are similar for motorhomes. Just add a few hundred on the pricing.
Anyway, for what it's worth:
We found that taking UK routes were no more beneficial and possibly a little bit more expensive than direct routes.
The factors you need to take into account are as follows.
UK OPTION: 1. fuel to irish ferryport;
2. ferry to uk;
3. fuel to uk ferry port;
4. ferry to france;
5. fuel to final destination
DIRECT OPTION: 1. fuel to irish ferryport;
2. ferry to france;
3. fuel to final destination.
The main areas we found to make the UK option non-viable were:
A. The exorbitant fares between Ireland and UK.
B. Your landing point in France can add as much as 50euro to your fuel bill as you are that bit furhter North.
C. The time and hassle of driving through the UK, personally we would rather spend that time relaxing on the ferry to France and driving in France.
To help you calculate the above details we find the following sites helpful.
For ferry prices: aferry.ie
For fueling prices: viamichelin.co.uk
and http://www.drive-alive.co.uk/fuel_prices_europe.html