You might be overthinking this.
Look back over your relationship and see if you find a pattern, no matter how "small," of tit-for-tat behavior on his part. Has he ever gotten you something nice, something you can't afford for yourself right now, and pushed for anything in return? The crudest variant is, "I gave you Nice Thing, now give me sex," but there are many subtler versions possible.
Has he ever, however subtly, made you feel that you "owe" him for any largess on his part?
That's what you need to think about. If he has simply been generous because he's a generous dude...well, go on the vacation! If you're planning life together with him, or anyone!, you need to understand that relationships won't always be 50/50 at all hours of the day or night. If he loses his job, are you going to move under a bridge with him because, welp, he can't pay for half the rent anymore, so...there you are?
When you're sick, are you going to insist that he take up none of the slack in your daily lives, 'cause 50/50!, and just let the dishes and laundry pile up until you're better?
What if your post-university job only pays you, say, half of what he makes? Will your boyfriend (soon-to-be-fiance????) have to live down to your means?
See how controlling on your end this starts to get? Now, understand that I'm NOT saying you are controlling--you seem to be an exceptionally lovely and together young woman! Your instincts are ABSOLUTELY right about sharing the load! However.
However. Try this: expenses are split proportionately. Everyone gives according to their means (we're Old-Fashioned Bolshies here in the Springheeled-Jill household!). Let's say your future job gives you 50% of his income; this means that you contribute 25% of the household expenses. This is fair.
Whether he thinks you will "owe" something in return for his generosity or not, you seem to believe that you do. Why is that? Growing up, did your family only give you nice things with strings attached? Did boyfriends pull that crap? It only needs to happen once or twice before one gets very wary, yes? (And rightfully so! Is there anything as maddening as someone insisting that because they gave you something, they have "rights" to be intrusive? I mean, fuck those people!) (Source: I may have had some experience with this. A little. Dear god, fuck those people!)
You know what? Find a therapist who specializes in relationships and book a session or two, just to unpack why your bf's generosity gives you the willies. It could very well be that it gives you the willies for a reason, and that reason needs to examined and judged. Also, learning how to communicate more effectively would be a good goal here. You've been tying yourself up in KNOTS, too frightened to address the issue about the gifts (why? Another subject to be addressed in therapy!), and now you are here again, afraid to talk to him directly about your concerns.
That's okay. Just about every human being on the planet could use at the very least a brush-up course on effective communication. Good communication--and boundaries!--are so very, very important to a healthy relationship!
So. 1) Examine your relationship closely, be honest; maybe ask a friend who you trust to be objective here: is there something controlling about your BF's generosity?
2) If not, perhaps propose a percentage of the expenses (what's your current income-to-income ration? Let's say you pull in 10% of his salary. Could you afford 10% of the trip?).
3) If it still makes you feel uncomfortable, it's okay to say no. Learning to say "no" is crucial to being an adult.
4) Therapy to figure out what makes you jumpy around generosity--fear of control? Feeling unworthy? These are valid reasons. You are allowed to feel this way. But if you can feel better, why not go for it?