I just did this to my trailer. You are going to like the Dometic toilet. It was quite an upgrade. I had the same issue you had and solved my problem by getting the 90 elbow similar to the one in your picture. My supply line lined up nicely so all I had to do was to measure and cut the line, slip it into the fitting and hook it up. A rather simple project.
I was asking around and someone suggested this...
smkettner wrote:
I would cap that blue pex line with a push-on valve fitting similar to above pic. Then use a flexible supply line from valve to fixture. Same as you have at home. Lowes/Home Depot has it all.
I suppose this would work if your supply line was far away from the new hook up. It would save you having to buy a bunch of fittings. I also wanted to preserve the shutoff valve but didn't see the need. If I am having trouble I can simply turn off the water pump and fix things.