Out of Whitehorse, top up with fuel at Otter Falls lodge, about 30 km before Haines Junction. The fuel is cheaper there than in the Junction.
Destruction Bay and Beaver Creek are a bit expensive. If you can make it to Northway in Alaska, that would be best. About 250 miles from Otter Falls to Northway, I think.
At the end of March it took me about 7 hours to drive Whitehorse to Tok in my CAR, doing the speed limit (well, 20 mph more than the speed limit in some places). Don't expect to be doing the speed limit in an RV. When we're with the fifth wheel or camper, it takes us more than 7 hours to get only as far as Beaver Creek.