To boost her immune system, you should offer lots of dark leafy greens (folic acid) You said you'd prefer no drugs, do you consider vitamin drops drugs? I offer my pigs and rats "sun seed" vitamin drops in the winter, it's got vit C and that's an immune booster.
I know what it's like, to not have the money for a vet, which is why we only have a few chickens and not the huge flock that some folks have. If we had a hundred birds, we'd lose our house paying for feed alone. If you don't mind me saying so, a little trick we've learned here is to buy meds from Lambert Vet Supply. It won't help your bunny, because it takes a few weeks to arrive via postal service, but there's a few things I think EVERY pet owner should invest in. Some antibiotics, the type would depend on what types of pets you have, we keep Penicillin, Amoxicillin, Ampicillin, Tetracycline, and Cephalosporin on hand year round when we can. (one bottle goes a long way) And LVS has the lowest price for Pen that we've found. *fish mox fish pen fish cillin* The stuff packaged for fish is non prescription and cheap. Whenever something gets sick -if we can't fix it ourself- we call our vet for advice and dose. Ivomec, the ultimate in parasite control. Costs a bundle but, it works and as with the antibiotics, it goes a long way. mind you, we can't afford to but all that at once, so one bottle here, one there... when the money is available.
I'm not telling you "don't take your four year old doe to the vet", I'm just offering some future planning advice. Honestly, she sounds like her best hope is with a vet. I know how the small animal specialist vets can feel like robbery so I do understand your hesitation.