Yeah I'm kinda in your same boat about finding it difficult getting enough calories on low-fat/high-carb. My digestion got so screwed up from my misdiagnoses and years on a high animal protein diet that I still get bloated easier than I should and I don't feel very hungry most of the time, regardless how much I eat. I believe my hypothalamus gland is out of whack, which is the gland that governs hunger.
When I first started my new diet path with the juicing then mostly eating salads/greens for the first month or so, pretty much following Dr. Fuhrman's "Eat to Live" diet, I lost a lot of body fat fast because Fuhrmans diet has you focus on nutrient-dense foods like greens which are a lot lower in calories than fruits and starches. It's no wonder that Fuhrman's looks the skinniest next to the other near-vegan diet doctors.
You got to aim at the most calorie-dense plant-based foods as your staple, like fruit, potatoes, or grains and try to pound them down. Use http://cronometer.com/
to help you calculate your calories.
After I heal my gallbladder a bit more, I might increase my fat intake to around 15-20% to see if I feel any different and if it help packs on some more pounds easier.
In Ayurveda medicine, I'm Vata dominant which they recommend eating more fats than the other two body types because of being a natural thin build and dry skin/hair and such. Would make sense.