I understand your problem & have 3 suggestions.

1. Your worry re vet visits. Is there a mobile vet, in your city, that provides a service to owners & their pets, in their own homes?
We have one in our area (bit far away for you, tho'. I'm in Brisbane, Australia).
It's called Vet Van & it's staffed by a fully qualified vet. Many people, for all sorts of reasons, use this mobile service for their pets.
Check it out under Veterinarians in the phone book. Or maybe your own vet would know.....or even do some home visits.

2. I don't know what your porch steps are like....how high etc. But some folk I know use, for their big dogs, those ramps that are used to roll motor-bikes in & out of trailers.
The ramps are not too expensive from auto supply stores. They're usually folding & fairly light. And the surfaces are made for gripping on.

3. If you want to hold a large dog steady while its going up steps or up a ramp....a huge beach towel (or a couple of them), looped under the dog's stomach (like a sling) will do that. This just takes a little weight off the dog & it can use its legs more confidently.