For new...I think the prevailing opinion for gas mileage is around 8-10mpg on a class C using the Ford450 engine. The only models getting 15mpg would be diesels which you pay for up front.
Discounts on new seem to run in the 25-30% range vs. MSRP on most popular brand class C's.
Check out MHSRV.com in Texas for some on line examples of list vs. discounted. Many dealers won't go that low...but it helps to have some perspective.
As to toy haulers...they tend to be a lot bigger than the 25-28 feet or so you need...Suggest you think about carrying the cart ON the hitch or on a swivel wheel behind the motorhome. Check tongue weight restrictions on any coach you are looking at as many have class 3 hitches without the full 500 lb. tongue capacity.
Also would consider small class A like the FR3 and others designed to bridge the gap between A & C.
Good luck hunting!