We had the problem at our first house when I was a little kid that the yard and damp, dirt floor more like crawl space than basement under the house for access was making a damp subfloor. With the wildlife and neighborhood pets wandering through the yard the fleas had taken over the yard and can live in damp flooring without biting your pets often enough to die from the chemicals. When the topicals first came out my mom was actually dosing every 2 weeks with advantage instead of monthly to keep them dead. I was too young to know what was used but that was the only time the yard was sprayed and it was with a specific product for fleas as well as the house and vacuum, vacuum, vacuum even with wood. Suck those buggers out of any cracks and dehumidify things if necessary. They don't like a dry house of any material. That's also when DE can be useful because it can cover the floor or ground and then be cleaned up or left to work into the dirt floor beneath a rabbitry while killing the hiding pests that aren't biting at the right time. If you want to get more serious there are things like sevin dust and sprays for livestock like poultry, which work as a broad insecticide and are actually toxic. It depends how extreme the situation is to warrant toxicity risks. I have seen fleas kill small animals when carried by dogs and cats into an area. Our apartment flooded because their pump failed and we had to empty the guinea pigs, degus, dwarf hamsters, and duprasi (fat tailed gerbils) to my mom's while they redid the floors. Well she's not good on flea treatment and despite not noticing it on her larger pets I had to pts the duprasi and hamsters within the week. Simply moving the animals back killed the rest of the fleas without a preferred food source to keep them going. Really once you get an established infestation in the area by wildlife and other pets treating the animals specifically is only half the battle and you'll lose the war. You need to kill those bastards on the ground or floors and remove all the ones you can so they don't keep getting on the animals or you'll have to endlessly chemically treat everything and still get some bites.
I had that same problem with the dang tropical rat mites. They could live on the pest mice coming in the house and they just kept re-infesting things. What was the worst was that if a mouse dropped a fed female in a gerbil cage hundreds of mites would hatch one night and I'd wake up to a bin of dead gerbils or in the middle of seizures and dying from bites no matter how many times I put revolution on them. It did no good to kill them after the bite and we actually took 3 cages, stacked them in the bathroom, and sealed the tops until the mites died before cleaning because they were so bad we couldn't handle the bedding and like I said it would happen over night. They'd be fine and the next day they'd be dead and if I stuck my arm in to check things it would come out completely covered in crawling dust specks that were mites. I ended up doing a layer of DE under their bedding and dusting them occasionally so the mites would die enough I could save the gerbils if it happened and spreading it on the floor around the cages of the gerbils and guinea pigs. The guinea pigs didn't get it as bad because I would guess they are farther apart genetically from mice or rats. It was a natural predator that we brought in that killed the mites after all the DE and chemicals and then we eliminated the pest mice after our 3 year fight against them and haven't seen a mite since. They couldn't live on my gerbils or guinea pigs in the house. They just had another source and were using them (and me) as an emergency food source to try to survive without their preferred host while failing to survive well enough to reproduce. I actually read of some people who just went ahead and moved because they had an infinite source of rat mites they couldn't stop. They said it was as bad or worse than bed bugs.
Fleas are a little easier and less dangerous than rodent mites but so common that anything could keep carrying them into the area. Right now I'd just try to keep the numbers down with cleaning and powders or liquid sprays depending on the surface and type of traffic around the rabbits, yard, and house while keeping the dogs and cats treated along with the rabbits as needed. If the rabbits actually get to the point they are suffering from the fleas instead of an occasional spread you can kill back off I would call a professional for the house and yard to nuke things at that point. If it gets that bad you've got to wave the white flag and while it's not without it's downsides you'll spend less having a professional use the right chemicals on the area than how often you'd have to buy milder products for spot treating areas and animals endlessly.
