I was curious about this so I read the original post to my DH, to get his opinion. His first thought was that the parents were tactless, if not hurtful by gifting their son with $400 of gifts, while giving DIL nothing. His second thought was that the OP should suggest using the gift voucher to buy a pressure washer or, if they already have one, buy lessons in how to use one!
We both agreed that the OP's DH needs to acknowledge the work she put in; yes, she may be at home and therefore have more opportunity for doing the work, but he at least needs to acknowledge that and show some gratitude.
However, while I don't expect the same value of gifts from my parents-in-law as DH receives, a $400 'just because' gift for DH and nothing for me at any time, would tell me where I stood in the family.
I think this is the crux of the issue for the OP. I don't believe the gifts mean squat. I think she has every right to feel some ingratitude from both her husband and ILs. It all boils down to the feeling of having a lesser standing in the family no matter how much you do to try to please them.
Both DH and I also agree with CakeEater and sparksals. I absolutely don't expect absolutely equal value of gifts for DH and me from his parents, but the blatant inequality of this size of gift would cause some unease. To be fair, if this did happen to us, DH would be uncomfortable too as his parents have always been very generous towards both of us. DH's take on it would be to use the gift vouchers for the desired purchases but then, as Two Ravens suggested earlier, use the money saved for those purchases to buy a treat for me, so that we both benefit.
(Extra info: DH's parents stayed with us a couple of weekends ago, I'm a SAHM and he ended up being away with work for the week prior to their visit, so I did 99% of the pre-visit prep work. DH acknowledged this and thanked me, both in private and in front of his parents.)