Historically in the Rhineland the region was rather docile and accepting under French rule, and wasn't particularly discriminated against. The percentage of conscripts who deserted was lower than the national average (well below actually, at 13% in 1804-1805 and 3% between 1806 and 1810, compared to 28% and 13% nationally), and volunteers equally around 15% of the conscripted rate represented acceptance of military service for France. Conscription rates by 1810 were around 40% higher than the national average, after initially having been well below during the first part of the decade. Business interests generally supported the French until late in the period, when the tightening of the continental blockade brought opposition. So too, most elite and notable interests rallied to the French, except in Cleves. Official administration ratios in regards to officials was roughly equivalent to "Old France" in the South : the Rhineland provided 4 prefects (and 2 outside of France proper in broader imperial territories), and had 15 prefects from the rest of France. In France as a whole each department supplied on average 2.3 prefects, while the Rhineland provided 1/1.5., and Southern France provided 1.7. So, mild discrimination, but one which was fading over time : by the end of the empire, most offices were held by local Rhinelanders and these were starting to head into the interior, but this process was cut short when the regions were lost. This discrimination was probably more because the Rhinelanders were provincials instead of not being "French". The Rhineland is famous for having a long-lasting admiration and support for Napoleonic codes of justice and administration, which outlasted the French period.
There was of course, some resistance, particularly due to continuing amity for Austria, and in Cologne in particular there was a great deal of opposition during the IIIrd coalition and during the Vth coalition there was a small rebellion in the Saar in response to the mobilization of national guardsmen, although this was put down in a few hours with no casualties. The Saar did in contrast have the best conscription record. There were continuing numbers of soldiers who served in the Hapsburg armies, and diaries of soldiers from the Rhineland do not seem to indicate that the French army was a "school of the nation" - the soldiers don't seem to have been very patriotic as Germans or French, but instead their identity was as soldiers. Opposition during the wars against Prussia was by contrast, minimal. Draft dodging was mildly higher than in the rest of France in 1806-1810, at 33% compared to 27%, but this was at least in part due to the Rhineland being on the frontier, making it much easier, and due to the many hills and forests. It would be unfair to say that there had been any significant change in the identity of the region to make them into French. But simultaneously, opposition does not seem to have been very severe. If there was opposition, it seems to have been in bureaucratic and legal terms to attempt to negotiate the position of the Rhineland vis-a-vis the French state, instead of violent military opposition. And neither was pro-German (and above all pro-Prussian) sentiment, strongly expressed.
German did continue to exist in administrative roles, and many of the mayors didn't speak French so relied upon secretaries for writing in French. In 1810 at least, mayors were allowed to write their reports in German.
As a synthesis of these elements :
1)The Rhineland wouldn't face a serious political opposition movement to being part of France. There might be occasional grumblings, but French rule was not markedly unpopular or detested, and in times of peace the burden of conscription and taxes will fall.
2)A surviving Napoleon France will have a different ideology of what is "French" and the boundaries of Frenchness as compared to the original France. While still being highly concentrated upon Paris and in this sense hierarchical and unequal, I think that it would be less exclusionary and more cosmopolitan. France is the greatest power in Europe and an empire which rules over many lands, and resultantly the imposition of French will be less defensive, and although encouraged by the French state, it won't be tinged with the fear of inadequacy and reactive nationalism in response to the Prussian/German challenge. Furthermore too, it will be more of an imperial conception, rather than strictly national conception. In time this distinction of empire vs. nation will close, but I think that it would be less tight under Napoleonic France than originally.
3)Nor do I expect that there would be any negative stigma attached to German on the French as compared to OTL. OTL German was a threatening language spoken by a dangerous continental enemy, the usage of which in the border regions carried distinctly political overtones inimical to the French nation. In contrast, in a Napoleonic France, German is spoken by a wide swathe of allied (and harmless) states in Central Europe, as well as admittedly by the more dangerous - but also distant - Austrians and Prussians. Rather than being a language which is to be suppressed, it is a language which is a useful tool for connection to the Eastern marches of France, and instead of a dangerous and threatening tongue, one which doesn't carry a negative symbolism.
4)I therefor expect that the Rhineland will see a steady expansion of French as part of France, as a result of education, connections to the rest of France, links with the broader world (which will have French even more markedly than the OTL 19th century as its greater lingua franca), ambitions to rise higher in the administration, and movement into the territory of other francophones, but that there will be less of a hard language barrier than OTL. "Frenchification" will still occur, but it will be on a frenchification which will be less absolute and which has less defensiveness against local languages than OTL.
tl:dr I think that the Rhineland would be successfully frenchified in regards to speaking French and being a loyal part of France which considers itself French, but that this would be part of a French nation with a significantly different perspective in regards to minority languages in general and German in particular, and with a different political ideology distinctly different than the OTL Republican ideology.