The ones that physically fit the space available. I guess you want to mount them flat on the RV roof. And im sure theres lots of things scattered around the roof like air vents and ceiling windows and such.
Ideally you would find a panel that fits the space, either as a single panel, or multiple of the same panels. Since then you can put them in series onto the same MPPT.
If thats not possible or not practical, you might find two types of panels that fit, for example a 100W and a 200W. Now most likely they wont match with either voltage or current, so ideally you would use an MPPT for each.
That is mostly given by the panels you want to use and the battery voltage. So firsst you need to decide on the panels. Then input that into the MPPT calculator
Likely, but that depends on how you plan to mount the panels. Ive seen people glue them on using adhesive out of a caulk gun, but im sure those will only ever come off again including the paint. A common way is to use mounts like these
Depending on the age of your current install, you might also need to exchange the roof penetration, due to grommets being brittle and disintegrating upon removing the old cables.
You might also need some PV cable cable extensions, but many RV shops also carry PV cables by the foot nowadays, so they might be able to supply that
