Hi, welcome to the forum. First and foremost, you are not going crazy. Having doctors tell you that they can't find anything wrong with you can certainly make you feel crazy, but you are not. It's not all in your head, you have a legitimate physical illness, so don't let them tell you otherwise.
Secondly, there are a fair number of overweight Crohnies, it's less uncommon than you'd think. The illness presents differently for everyone, and although it's more common to lose weight/be thin with IBD, it's definitely possible to be overweight and have Crohn's. So also don't let that get in the way of a diagnosis.
What tests have you had done besides the colonoscopy 5 years ago? I would recommend you have another colonoscopy, with lots of biopsies, if it's been that long and particularly if you're having new symptoms. With the exhaustion, have you had bloodwork done? Deficiencies in things such as vitamin D, iron, folate, and vitamin B12 can commonly cause fatigue and are very common with IBD. I had an iron deficiency last year which made me horrendously fatigued, but fortunately iron infusions made a world of difference for me - so it might be an easy-ish fix for your fatigue as well. Definitely get that checked out.
When they found the inflammation 5 years ago, did they give any indication as to why you were inflamed? Inflammation doesn't happen with IBS so I would be questioning that. I'd also be questioning why the doctor has recommended that particular diet - have you tested positive for any food allergies or for celiac disease?
If it were me, I'd be pursuing another doctor (it sounds like this one you just saw is brushing you off if he diagnosed you with IBS without doing any tests?) and I'd be pursuing a lot of testing. Blood tests, stool tests (fecal calprotectin), another set of scopes, and imaging tests (either CTE or MRE).