3 cats, 1 smelly problem. Please help!

[CindyM272]

Hello,
This is my first post and I am at my wit's end and would really like some help. My 3 inside cats, Chunk, Kevin and Rudy have had on and off loose stools for about 10days/2 weeks. It has the consistency of oatmeal, the smell is as far as I can tell, unpleasant but normal and there is no blood and little water. They defecate in the box and outside it, seemingly at random and it doesn't matter what condition he box is in, time of day etc. They do not urinate anywhere that is inappropriate. Near misses I can understand (Kevin often stands in the litter box, sometimes forgetting that his butt did not make it in) but we find piles of poop everywhere, daily. I am frustrated because my house stinks, I am loosing sleep over what is wrong with them and I feel sorry that they obviously have something wrong with them and I cannot help.
They are 7 months old, all male un-fixed litter mates. Their mother was a short hair Manx and their father is a local feral domestic long hair. They do not go outside and have not associated with the outside cats (we have 4 other "big kids" that are usually in & out, but as to not spread anything that may be wrong, they are strictly outside for now) for about a week. They have 2 boxes, one is just a tray and in the corner of the kitchen and the other is covered and in the living room. Their food and litter are both the same brand as always. The food and water stations are not near the litter boxes and everything has been in the same place for a long time. They are not vomiting and there are no worms. The litter is cleaned morning & night, and it is dumped and brand new litter is put in every week. Aside from the odd kleenex (I think they think they are mice - or human mucus is kitty crack...lol. All bins are out of reach) I have never seen them eat anything inappropriate - I'm a bit of a "helicopter mama". Their coat is normal (Chunk and Rudy get some messes stuck on their rear on occasion as they are long hair and we wipe it off with paper towel and water so they are not re-digesting it time and time again), eyes and ears are clear, gums are a normal pink colour.
Chunk seems to be the worst culprit - he cries to go in the basement (why I cannot tell, he's never been in the basement so has no associations with it whatsoever) and if we try to cuddle him/distract him, he runs around like a crazy thing, pooping as he goes. We now just let him go down as to 'soothe the savage beast' - it seems to work for the running and pooping issue, but the problem still remains as to why he and his brothers have such weird attitudes to the litter box.
I tried a fast for 24 hours, which helped (the stool was regular tubular shape - but I found it in the spare room *sigh*), but clearly I cannot just not give them food. They are fed breakfast and supper and I have cut them off of treats/human food scraps since this began. NONE of the suggested solutions I have read about have worked.
They are otherwise happy, healthy, sweet, loving little guys.

Our house does not have a spare bathroom and all rooms are occupied - my only method of re-training is to close them in a large dog crate (the biggest kind you can buy) after mealtime and during the night. I have not done this yet.
Money and ethics are a big factor - I am prepared to change their diet within reason. Since we are feeding 7, we have to buy No Name brand as funds do not allow best quality, but as of today, we are switching them to a better quality name brand one. I would very much prefer not to have to get them to a vet who will undoubtedly give me drugs and a large bill. I would *much* prefer something natural.

Is there anything else I can do? Will yogurt work? How about a protein top-up of eggs? (we keep hens so have plenty) How about a diet of real meat - again, we can raise more chickens for cat food, no problem.

Thank you very much