My dog is really itchy and distressed. I'm at my wits end. Any advice would be great

Hi all.
My 2 year old Jug, Bella has developed a horrible condition over the last few months.
She's severely itchy, to the point that she's given herself sores and scabs. She often wails and crys because she's so distressed.
It's costing me a fortune in vets fees. They keep saying it's a flea issue. Yet they've never found any evidence of fleas on her.
She's flea treated monthly and I've had my entire house treated.
She is always trying to get to her back end and this is where most of her sores and scabs are. She has her anal glands emptied.
She is always knawing at her front paws and dragging herself along the carpet.
She's fed on wet and dry food.
I've tried shampoos for itchy sensitive skin etc but I'm getting nowhere.
I went to an independent pet shop and the owner suggested that she could be gluten intolerant. Does anyone have any experience of this? Her main issues are
Itching
Sores and scabs from scratching
Biting paws
Hot tummy to touch occasionally
Over full anal glands
Very soft poo.

She's just not herself either. I'm getting annoyed at keep being fobbed off by the vet.
Any advice would be grateful for.
Sorry for the long post.