A starter kit isn't really helpful anyway. We got a 4-pack of RV toilet paper and a super cheap sewer hose (gee, wow). We gave the hose away. :(
The things mentioned by others will get you going or are a should-have (like the EMS). Many things are what you figure out along the way. Some are things you see in an online catalog or in an RV parts store that you just gotta have.
You *could* survive without these but am sure you're you going to want them sooner than later: BBQ, folding camping chairs, outdoor mat, regulator (if on city water), filter(s), road maps (if travelling around), some basic hand tools. There's probably a few more.
Don't forget the basic necessities on the first trip out like toothbrush, coffee cup, soap and kitchen utensils (BTDT). In the longer run, there's all kinds of great/cool stuff you can get like an RV GPS, memory foam mattress, stabilizing struts for the stabilizer jacks, laptop & mobile mifi hotspot and much more...
Tip of the day: Do NOT buy RV toilet paper with it's mafia-inspired pricing. There are numerous household brands that disintegrate in water and are the same thing. We use Costco Kirkland brand.