The HK45 is pretty darn similar. It uses most of the same internals. I've used USP instructions to take mine apart...
My question is, are the internals on all of them the same? I have an hk45 and got a pretty good bit of realy fine river silt/sand in it which needs to come out, and all the dissasembly videos I can find are for either the usp or hk45c.
You probably don't need to fully disassemble the gun, get some canned air duster and just do your basic field strip, blow out the grime if anything is really caked on get some synthetic safe Gun Scrubber and go to town with it, let the gun dry, reoil the springs etc.
The slide you may want to open up and pop the extractor out and make sure there's no build up, maybe the firing pin as well.
If you do take the gun apart past the basic field strip, make sure to take out the trigger bar detent and spring well before the instructions call for, those parts likes to go flying.
HKPARTS.net has USP armorers manuals for sale, I've got it on order and should hopefully have it Tuesday, if it ends up being worth it, I'll let you know. Some of the stuff I've downloaded from HKPro was all older scans and kind of rough, hoping this will be a better product.
As for USP and Hk45 parts similarities, the triggers, sear parts, springs (minus the recoil spring if I'm not mistaken) are the same. Some of the detent plates may be the same, but not all of them are and there are some slight differences in the safeties.
The take down instructions for the guns are a little different as the mainspring (also called hammer spring) assemblies are a little different but it's not that hard to figure out.