berreez,
the difference in routine maint costs are minimal. unless you buy one that's old enough to need the engine or tranny rebuilt, the difference in oil/lube/filter, coolant & tranny fluids, and tires are peanuts in the big picture. anyone who eliminates a diesel pusher as an RV option because of maint costs, was just looking for an excuse to do so.
as for your 50k budget; I own 3 modest homes, 3 RV's (including a diesel pusher), 3 trucks (including a diesel HDT), a Jeep, a Harley and an ATV; travel all year; and my annual budget/actual costs are around $52K for the last 5 years. Unless there's more to your story, you should have no problem at $50k