I have a Hustler amateur mobile antenna model RM-80 for 80 Meter 25-30 KHz.
Can
anybody tell me the length of the counterpoise needed for this model?

A 1/4 wave counterpoise would be about 65 feet or 20 meters. Mobile antennas
use the vehicle body as a counterpoise. Your 65' wire or a vehicle used as a
counterpoise will not produce very good results, because the efficiency of the
short radiator is only a couple of percent.
As an option, a really good performing portable 80m antenna that I have used
many times is an inverted vee supported by one of those 33' fiberglass poles.
It is fed with 450 ohm line and a tuner. The vee is 50' (15 meters) per leg.
You will need about 80'-90' to spread the vee. That's a lot of space, but you
will need 65' for the counterpoise for the Hustler. It will beat the
Hustler-counterpoise idea significantly, and you can QSY all over the band.
With a Hustler you have a narrow bandwidth.
Good luck and DX.

73 Gary N4AST