A cheaper and just as effective means of raising the rear end is air bags. Air bags are a DIY job, where as changing rear springs is not something most DIYs can do.
I've had air bags under my 95 Bounder for about 8 years. The ride is even slightly improved.
I suspect Winnebago got the vent wrong and it acts as a syphon. The vent tube is inserted too far into the top of the tank and it also needs an opening going up to stop the syphoning effect.
Is you generator a BGE on NHM? I may have the service manuals for them. Tag on the operator side of the generator with the info I requested.
Richard