I plan on re doing all the plumbing in the camper if we decide to keep it and not upgrade it. What I am doing is also the same and getting rid of the hodgepodge of plastic fittings and useless 1/4" lines and unreliable cone seals and switch to pure pex with proper brass connectors. this also involves changing things like taps and such out to proper residential ones but the benefit of that is you won't have as many issues with freezing in the winter and proper pex will handle the freezing but it's usually the plastic connectors in rv's that die. Im not saying I won't have to winterize but it would let me just use air and if a tiny bit of water is left in a line somewhere it shouldn't cause an issue
Your best bet is to try to build it in sections and pull it into place so you're making final connections where you have more space but that's only if your layout allows that. Otherwise look for the smallest crimper you can find. you will only need a 1/2" one so look for something like
THIS