The immune system basically is only effective at doing its job and responding when needed to threats when it is strong and not over stimulated, or suppressed. It can become a careful balance as a result to keep it strong, but modulated (not hypersensitive).
Allergies for example - are the biggest example of a hypersensitive immune system - an immune system that is so over active, it over reacts to things that are not true threats.
Dr. Incao still has the best write up to explain how vaccines work and over stimulate the immune system. Keep in mind that this is what vaccines are designed to do - stimulate the immune system, but no one can offer guarantees on how each immune system will respond. We know at this point, that Beagles and Pugs do not respond the way Berners do as Berners are immune deficient and highly prone to auto-immune disease. Take a look at this article, as well as others by Dr. Jean Dodds DVM, and Dr. Ronald Schultz to understand immune stimulation and over stimulation.http://naturalk9.com/PDF/How%20Vaccinations%20Work%20-%20Dr.%20Incao,%20M.D.pdf
In essence, we start with a weak immune system (breed genetics) - we add a vaccine load that they are not genetically equipped to handle (even just one shot can do DNA damage and over whelm the immune system). Then anything else that is presented to the neurological and immune systems to handle/process/or filter - will just be an added load. All foreign agents, including chemicals, pesticides, adjuvants, and toxins further burden the immune system - so Revolution, other flea and tick pesticides, heartworm etc. - not good on Berners as I have yet to find a healthy one able to handle this load (and I see a lot).
So I said what I did because it is clear that your vet does not know this breed - or would never have suggested Revolution on top of what could possibly already be an adverse immune response (the skin is referred to as the 3rd kidney, as it is responsible like the liver and kidney, for performing a filtering and detoxing function and so much of what we consume, comes from the inside out and shows in our skin). The addition of antibiotics are weakening on the GI Tract - they strip the tract in fact and 70% of the immune system is in the GI Tract (see other posts of all of the GI Tract issues from immune responses) and so it is really important that as soon as the antibiotic course is done, you begin administering probiotics right away, twice a day to replinish the beneficial flora in the gut and help repair the lining.
Another vet should be able to provide an opinion without pushing skin scrapings. I have three vets for example and none would have even started with a skin scraping as a matter of course in this case. Try to call around and find someone who understands immune deficiency, risk of vaccines, and can take a different approach to Tucker's case. They should listen to your history and everything that has been administered, dates of, etc. to help piece together a treatment plan.
You may also want to consider joining the Berner-L, the largest Berner discussion group in the world. I moderate a Berner list for people who feed whole fresh food (raw or cooked) and raise naturally without chemicals and pesticides - just so you can appreciate the issue in this breed, my NaturalBerners list has almost 1,000 members and is the single biggest Breed specific natural rearing list on the web - there is a reason for that