That's a really crummy situation. And yes, no doubt, they are treating you like you have an ED and basically treating for one.
I know how frustrating it can be, but I don't know what you could do to "convince" them you don't have an ED. Any attempt to try to convince them you don't may be interpreted as evidence that you DO actually have one. So, it's almost a lose-lose situation, it seems.
IMHO, the best thing for you to do is to absolutely cooperate even when they're treating you for an ED you don't agree you have. Rant about them here, think about all the bad stuff you would like to say to them in your head, be angry at them - whatever you feel at the time; but don't show it to them. Make them believe that you are taking in everything they say, and don't fight them. I know that this is so hard to do when you truly disagree with someone, especially as it's YOUR care, but I think that it's the best option, for the time being at least.
Are you able to meet with a psychologist? Specifically, one who specializes in eating disorders. Go as an outpatient. Tell him/her everything 100% honestly. Tell them you are there because you considered what your medical team has been telling you and you wanted another opinion as you don't feel you have a fear of gaining weight but are open to discussing the situation as it stands. Don't be insistent that you absolutely do not have an ED or that you're doctors misdiagnosed you and they are wrong, etc. even if that's how it really went down. Come in as an open-minded person who is taking initiative to put her best interests in front of her and who wants to evaluate whether what her doctors said is consistent with a psychologist's perspective on her situation. Be sincere and express that you need help figuring this out, whether or not you really have an eating disorder, because it's gotten to be a bit of a messy situation. Basically -- you need someone you can trust, who you can tell the whole story to, who you can talk to. You need someone on YOUR side, who has power in the health-care system. A good psychologist would have no reason to think you are an anorexic trying to trick them into declaring that you don't have an ED if you are open and sincere and if you really don't have anorexia. They can communicate with your current healthcare team and help you get the care you need, and not get care you don't need. At the same time, your healthcare team will see that you are taking initiative and considering what they said. What's more, it's always a luxury to have a professional to talk to, eating disorder or not; it's not a secret that you've been having a hard time lately, from a chronic illness at that.
I really hope you are able to do something to get out of such a $itty situation!
As for your diet, you must try to gain weight, even if it's just to please the annoying doctor.
I am not one to say if you do or do not have an eating disorder and it's certainly not in my place to do so, but it does seem to me like you have some disordered eating or just a somewhat unhealthy relationship with food. That said, I have very strong opinions about the way EDs are handled and I hate to say this, but, there is no doubt in my mind that the way they were/are treating you for anorexia is a HUGE reason for your current relationship with food. I want to say it's their fault, maybe not entirely but putting that kind of pressure and emphasis on someone absolutely does not help. Add that to someone with Crohn's disease who is more likely than the average person to develop a fear of eating just because of the pain they know will come, and, well...it's just not at all conducive to a healthy relationship with food.
As for gaining weight without high food volume, you really need to use those calorie-dense foods. Have a 1000kcal shake between each meal. Does nut butter digest ok for you? If it does, use it. Use cream combined with milk instead of just milk. Put oil or butter on whatever you can. You can easily fit in 5000+ kcal with something like this:
B: 2 pieces white toast with butter and jam or cheese or nut butter or avocado, a pot of full fat yogurt (some brands are 250+ kcal for a pot), a glass of your choice of juice
S: A 1000 kcal shake
L: A bowl of [vegetable, chicken, whatever] soup pureed with 1+ T oil added (while blending to emulsify), 1 cup of white rice cooked with broth and oil or butter, chicken or another meat you like; can use sauces to increase kcal intake, etc., a diet soda
(you can still drink diet soda!)
S: A 1000 kcal shake
S2: Some type of nutrition bar eg. Cliff bar, etc. should be ~250 kcal
D: Mashed potatoes made with cream and butter or pasta with butter or something similar, salmon or your choice of fish or meat, a small serving of vegetables of your choice made with oil or butter, a cup of your choice of juice or milk (better if 1/2 milk 1/2 cream)
S: Dessert -- a piece of cake with tea or ice cream or hot chocolate made with cream or any other dessert of your choice
S2: Ensure or your choice of nutrition drink (the type with extra calories) before bed
Sandwiches work well for lunch too and can be made to be very calorie dense.
Does that look like something you would be able to do, or is that still too much volume?