Here’s an affordable lock that you can use on a kayak:
Master Lock 8413DPF Python Cable Lock with Key, 1 Pack https://smile.amazon.com/dp/B00006407M/ref=cm_sw_r_cp_api_glt_fabc_XCJ3KNT9BPHKZ4XF1YQB?_encoding=UTF8&psc=1
I carry two of these when I’m traveling, one on the bikes and one on the kayaks. Actually when I’m traveling long-term I tend to just bring one kayak for simplicity; I actually don’t recall whether I’ve been able to lock two boats at once with this, but it should be possible with a little creativity.
Another plus of having it on the back hitch of the RV is that it shakes the RV if it’s tampered with. So that should wake you up. An audible alarm disk lock would work too.
And it leaves the bed of the truck empty for a generator (like if you don’t have an onboard gen and need to stay in a truck stop one night where it’s not convenient to leave it out, other vacation stuff).
A Steel cable lock like this is really tough to cut without using something like a grinder or saw. Normal bolt cutters won’t do much against rubber coated, braided steel cable. The link below is similar to the ones we used.
Cable Lock, Python Adjustable Keyed Cable Lock, 6 ft. Long, 8413DPF https://www.amazon.com/dp/B00006407M/ref=cm_sw_r_cp_api_glt_fabc_CPC424MJRHP4VQF3XFDF