Both Dave and Via2010 gave very good answers to the OPs questions. My contributions are:
1. I have no idea where one can park an RV. Some towns do have camping grounds where you can stay. Suggest you search for an RV guide to northern Spain, they DO exist. You may have to go several km away from the Camino route proper to find such a parking place. Along most Camino routes, secondary or tertiary roads typically weave along the Camino. A GPS or smartphone with Google Maps is useful. Reception is usually great.
2. Your wife can get stamps in a credencial the same way you do. You only need one per day east of Sarria (on the Frances) or beyond the 100 km threshold. This is 200 km if bicycling. So, just call it 200 km for the both of you. Once you arrive 100 km out from Santiago, or at Sarria on the Frances, you must get at least two stamps daily in your credential. Technically, this applies to bicyclists too. The purpose is to ensure that you actually followed the route and made continuous chronological progress. Of course, this will be easier for you as you will cover the distances easily.
3. Vegetarian options and choices exist. They are not usually flagged in a cafe menu as vegetarian options, so you must learn how to read a menu. Learn to use Google or Bing translate.
Search in the Forum using the search function in the upper right hand corner.
But, be aware that, in Spain, "carne" (meat) usually means red meat. So if you ask for something 'sin carne' (without meat), do not be surprised if you receive a dish containing chicken (pollo), fish (pescado), shellfish (mariscos), or ham (jamas). These are NOT considered meat strictly speaking.
Better to explain that 'soy vegetariana (female veg), y no comido todas carne...' ( I am a vegetarian and I do not eat any / all meat...)
4. I know of pilgrims who choose to sleep rough, but who pay to stay at an albergue just for the showers and meals (where offered). I understand they prefer to drink or smoke dope (whatever) or just prefer their own company. So, yes, you should be able to pay the going price for a bed in an albergue but only use the showers and meals facilities.
5. True, there is an alternative (aka tourista) "Certificate of Welcome" (Certificado de Bienvenido) issued to tourists, people who say they are doing the Camino for sporting purposes, and other people not otherwise eligible for a Compostela. It is a similar-looking document to the Compostela that welcomes the traveler to the Cathedral and thanks them for making the journey to venerate the Apostle Saint's relics.
I recommend presenting both credencials with stamps, ask for your Compostela, and then tell the person behind the counter that your disabled wife drove the same Camino to accompany you, and visited the Cathedral. Ask for a Certificate of Welcome for her. That should avoid any unpleasantness or frustration.
I hope this helps.