Travel insurance - don't leave home (or even plan a trip) without it. Perhaps check with their credit card company, if the deposit was put on that, to see if they offer any type of travel cancellation reimbursement...
The proprieter is not necessarily being totally out of line, after all he probably will not be able to re-rent the motorhome for that time period, so he is out that money. If he offered to just switch the rental, he would be displacing a future potential rental just to replace the money he lost.
Super advice!