So, you want something light and reasonable size.
If it was me, I'd get this antenna:
EM Wave EM-M43002 Tri-Band 150-162/450-490/763-870 MHz Mobile Antenna - 17.7 in - NMO - Spring Base
theantennafarm.com
I've used that on my work truck and it's a good antenna.
And I'd get this base station adapter to provide the necessary groundplane:
*MBCN pictured with optional antenna (not included) Manufacturer TE Connectivity Model # MBCN Product Description Mobile to Base Station Adapter Kit Product Narrative Converts mobile antenna to a base station. Use with NMO style HF, VHF and UHF antennas. Includes bracket, clamps, radials and...
theantennafarm.com
You'll need a suitable length of coaxial cable to get down the mast and into your RV. Times Microwave LMR-240 is a good, low loss cable that is light enough for this sort of application. I wouldn't use anything larger:
Build a custom coax cable assembly by selecting cable type, length, connectors, and heat shrink. Made to order by The Antenna Farm.
theantennafarm.com
Make sure you secure the coax cable near the top. You don't want the weight of the cable pulling constantly on the antenna base.