The idea here is that veganism is so unhealthy that obviously one could never promote such a diet without being a shill
I haven't actually run across any doctor that provably has a conflict of interest in the direction of veganism. One can always unfalsifiably claim "Study X would have found that veganism is bad if not for the interest of the people leading it", but without any clear connection, one can't dispute such an argument.
The best that has been come up with by anti-vegans is that in all the organizations that agree that "A well-planned vegan diet is nutritionally adequate", either one ancillary advisor is an adventist or went to an adventist university or the organization worked with some vegan organization for advertising their claims to vegans. Again, if there were some actual corruption they would demonstrate that, but at the moment no such evidence has been presented.
Meat production in the US is highly subsidized by tax dollars, and consumes much more resources than otherwise vegan alternatives. So if there were a "big tofu", they would also be vested in increasing animal agriculture since most soy grown worldwide is fed to farm animals. Same for resources like water or grain. There definitely is "big beef"; there is no "big vegan".
Would this paper be an example of shilling? Particularly the part where "This study was funded in part via an unrestricted research grant from the Beef Checkoff, through the National Cattlemen’s Beef Association. The sponsor of the study had no role in the study design, data collection, data analysis, data interpretation, or writing of the report." Or this one, which has negative things to say about vegetarianism in kids, funded by, again, the Beef Checkoff. In some cases they are conducted directly by this organization. What is this Beef Checkoff? Per their website, "The Beef Checkoff program is a national marketing and research program designed to increase the demand for beef at home and abroad. This can be accomplished through initiatives such as consumer advertising, marketing partnerships, public relations, education, research and new-product development." Or this one funded by Astrazeneca, makers of Crestor.
This is from just poking around for a few minutes. Someone ought to make a more complete list (most articles have funding within the full text that is typically behind paywalls). If someone puts a more comprehensive list online somewhere I'll link to it.
Here is a list of authors I have found that appear on papers funded by cattle/dairy-promoting think tanks and organizations and links to the respective articles, so that you can ctrl+F for them if their work comes up: