RV sliders windows can be hard to open or close for a variety of reasons.  All of the following should be investigated before ordering a replacement window:

Clamp ring roll - Radius RV windows are usually installed using an inside clamp ring. This part is screwed together with the RV window frame, clamping the wall between them.  The clamp ring varies, depending on the wall thickness specified by the manufacturer of the RV. The wall thickness must accommodate the actual wall materials, plus whatever sealant is used under the outside RV window flange. If the clamp ring use is too thin, there will be a gap between the clamp ring and the inner part of the RV window.  The application of the mounting screws will cause distortion of the RV window frame, resulting in a hard sliding window. This situation can usually be remedied by installing the proper clamp ring.

Wrong mounting screw - The correct RV window installation screws for most Hehr RV windows is a #8 x 5/8 Phillips head. Use of a longer or large screw will affect RV window sliding by distorting the track, which the opening pane rides on, this situation in not remediable & the RV window must be replaced.

Damage to the window frame - Dents in the frame causes the slide channel to be pinched, leading to increased slide resistance. Visually inspect the frame surface for signs of distortion, if distorted, use a narrow block and mallet to gently tap the frame back into alignment, check for improved operation.

Frame interference - Check for clearance between the sliding pane & the mullion. Mullions can be twisted due to impact or improper assembly.If the mullion/slider space seems inadequate, try rotating the mullion to permit greater clearance.

Damage to track - All sliding panes are captured in a plastic or soft rubber track within the RV window channel. This track can become damaged or distorted & may cause hard sliding, usually it is impossible to feel down this track. In order to check for damage, drag a small flat blade screwdriver along the top & bottom tracks. Any resistance will result in increased sliding resistance & the damaged track needs to be replaced.  Please see the instruction on glass replacement.

Debris - If the tracks are not properly cleaned upon completion of the RV, debris, such as shavings may accumulate which can impede proper sliding action. This can be alleviated by blowing the tracks out with compressed air, vacuuming out.

Oversize window/Undersize hole - Proper installation requires that the rough opening size exceed the window by 3/8" in both height & width. If the hole is too small the RV window may have been forced into place, distorting the frame & causing the sliding pane to be squeezed, thus increasing resistance. If this is the case, the RV window must be removed from the wall & the rough opening expanded to allow an interference free fit.

Glass too this/Track too thin - This can occur, due to improper parts selection at time of manufacturing, the remedies here are: replace the track with one to accommodate thicker glass, replace the sliding pane, or replace complete RV window.