Long hose...short hose-----doesn't matter. That has NOTHING to do with overfilling your fresh water tank
Only 2 ways water goes into fresh water tank
Via the 'normal' fill (either gravity fill port OR 'FILL' valve)
OR back-flowing thru pump via internal check valve
SO if you have a gravity fill port...........then pumps internal check valve is allowing back-flow
IF you have a 'Fill' valve ---it could be leaking thru, not closing OR pumps internal check valve
If you have a 'Fill' valve you can disconnect suction line at pump and then using city water see if water back-flows out of pump.
No back-flow then FILL valve is problem
Back-flow then pump check valve is problem
With fresh water tank full....secure city water and turn on pump. Use up the tank allowing flow thru pump to flush any stuff off pump check valve
IF 'Service Folks' can fix it---then it's because they do NOT want to or they are NOT someone I would want doing ANY repairs
Overfilling fresh water tank is a very common issue.....pump check valves and/or fill valves being problems is common
This can't be the first time these 'Service Folks' have had to deal with it