I once owned an 89 excella.
The existing pipes run across the top of the fresh water tank and are probably being squeezed by the tank. You will likely have to drop the tank down a few inches in order to get the pex shoved through.
If the poly lines are still good consider a polybutylene pex adapter fitting on each end.
Steve, Christy, Anna and Scout (Border Collie)
1994 30'11" Excella - rear twin
2009 Dodge 2500, 6 Speed Auto, CTD, Quad Cab, Short Bed
Hensley with adjustable stinger
WBCCI # 3072
AIR # 6035