Firstly weight gain is majorly dependent on your metabolism and genetics play a definite role too.
You have to be patient and give your body time to gain a healthy body weight especially also since you have arthritis.
Eat meals at frequent intervals.
Eat a mid meal between each main meal.
Avoid junk foods.
Include dairy, sprouts, fruits vegetables in your meals.
Avoid alcohol and smoking.
Exercise at least thrice a week whatever is possible and comfortable.
Even if you were to take medicines or supplements. They will not work in isolation. You have to have a and nutrient dense meal.
It is very possible that you are not eating the right foods at the right time. Also just
eating two or three very heavy meals is not going to help much. You have to balance out your intake to manage stamina and energy levels.