Seren,
Many people have had qualified welders design and fabricate beefy single, dual and triple receiver hitches for their Heartland 5th wheels in order to carry bikes or small platforms. I've never heard of any of these causing issues to the RV. Our frames are not designed to tow trailers, motorcycles, cars etc.
Since Heartland cannot control what retail purchasers will do with their hitches, we don't go too far in what we provide. As such, you'll find a 1-1/4" receiver on the back of your Landmark. Personally, speaking from my own experience only, I don't like putting bikes racks on 1-1/4" receivers. I just see too much bounce and I have actually had bikes bounce off the racks. There are ways to tie the bikes in better than maybe I've done. And I know many, many carry bikes on the receivers we provide and have no issue. I just like sturdier myself
If you plan to haul bikes, I wouldn't worry much about affecting your warranty.