I disagree with the comments saying age no longer deserves respect.
There are so many reasons that underpin this "old fashioned" value, especially in a family context.
First of all, parents have normally put many years into caring for their families. And with age comes issues like backache, joint pain, leaky bladders etc.
I realise many families on here don't have ideal family circumstances, and I expect this is where the inroads into the age deserves respect notion come from, but there is no suggestion this is the case in OP's family.
On the other hand, I think the OP is quite lucky that, at 30, her son and his gf want to come away with the parents for a birthday celebration, and still more so the single 28 year old. These are circumstances to be cherished and embraced by op, and I certainly would be being as welcoming as possible to the GF.
I find it a bit hard to relate to this scenario, as in our family trips with parents tend to be organised and paid for by the parents, which sort of gives a natural assumption they would take the best room. It is hard for me to figure out where the respect aspect stops feeding into that, and the financial and organising responsibility become the relevant factors.
I think in the op's situation - where she is neither paying, nor organising - I would offer the better room to the son and GF.
If they are well brought up, they will decline. But if not, then I'd take the hit anyway op. Not many families, it seems to me, would manage a trip away together in these circumstances, so if you value that, and especially as you don't have the wherewithal to be shouting them (which is fine: if you don't, you don't), I would be as facilitative as possible.
That said, I do think the idea that you don't respect your elders is, (outlying circumstances such as neglect or abuse excluded), an example of degenerative values in society that might feel modern and edgy but will result in a less decent environment for us all down the line.