To modify your current TH you will need to design with the plumbing, electrical, holding tanks and weight plus weight balance in mind.
I have made modifications to our 5th wheel so my 94 year old dad is able to come with us. I added a lift platform at the entrance door to get him and his wheelchair inside. We use a pottyseat next to his bed in the living room. I bath him in the handicap shower at the RV parks. I also have a Hoyer patient lift in case I can't lift him.
I've seen lots of modifications done to RVs to make them more accessible and there are companies (do a Google search) that specialize in it. It's costly so if you are familiar with remodeling techniques and handy with tools you can save money by doing it your self. First I would look for products that might help without the need for remodeling....the pottyseat, inflatable tubs, lifts and ramps...ect. These are relativly inexpensive and may solve some of the problems for now.