True, but lean mixture means not enough propane for the amount of air. Since you made no changes to the furnace then the logical suspect is a continued blockage causing less propane than normal hence the lean mixture.
I will be the first to admit that I sometimes jump straight to the weird obscure reason why something is not correct and ignore the simple causes. But more often than not the simple reason is what prevails. I would go with what Chris Bryant suggests since he's been repairing RV's for many years.